Because the magnanimous rightly think highly of themselves, they will therefore rightly demand that others do the same. After all, justice is an extremely important thing, and requires that everyone get their due; it doesn't include exceptions for oneself. The magnanimous expectation of being treated as great therefore is wholly consistent with justice; the great-souled are just demanding that justice be done in their own case. What is more, since they are the sort of people who deserve the best, that's precisely what they should demand, and this is itself a virtuous demand. Aristotle famously calls magnanimity the crown of the virtues, and it is easy to see why: it is the insistence of the heroically virtuous on the virtuous work of treating virtue as it deserves.
Perhaps the most important issue to do with magnanimity is the attitude of the magnanimous to the goods of fortune, things like wealth and fame; this will be the locus of an important difference between Aristotle and the much more Stoic Cicero. It is also the point at which the magnanimous man shows to greatest advantage, since if one doesn't look at it one might get the impression that magnanimity requires always demanding things of people. It should be clear from the description I've already given that Aristotle's magnanimous man is concerned with goods of fortune, although not mastered by them. The magnanimous demand great things, but they do not do so as beggars, but in an odd way as benefactors: they are merely giving others the opportunity and privilege of reward virtue. The goods of fortune, however, are not particularly important as rewards for virtue, at least on their own. Because of this, they prefer to give than to receive, because receiving puts one in debt, which is a position of inferiority, which is not an appropriate position for the heroic and highly virtuous. They ask for help only under genuine necessity, but they give help freely. They act loftily towards the lofty, because they will not place themselves in an inferior position, but towards ordinary people they do not put on airs, because that would be a vulgar attack on the weak, disadvantaged, or inferior, and would itself suggest some kind of inferiority. They tell the truth, do not hide their feelings, are openly contemptuous of anything that is contemptible. Their lives revolve around themselves and their friends, and no one else. They are independent, they do not whine at their misfortunes, they do what is required, and will prefer things of beauty, even if they are useless, to ugly things, even if they are profitable, because they are wholly unservile. And that is perhaps the best way to grasp what a Greek like Aristotle would have seen as excellent in magnanimity: is it is that virtue making a man wholly unservile, completely free from anything suggestive of slavishness, a master in his own right, even if rules nothing else but himself.
Aristotle's account of magnanimity focuses on justice. This has been interestingly unstable in the history of discussions of the virtue, and part of the reason is that the magnanimous man in Aristotle's account can easily be made to sound a little too concerned with whether he's getting what's his. This is not quite a fair assessment, because the magnanimous man does not have a slavish interest in goods of fortune. It is also true that the greatness of the great-souled is nothing other than the greatness of virtue, a point which is perhaps made a bit more clear in his Eudemian Ethics account of the virtue than in the better-known Nicomachean Ethics account. But the problem remains that one can question whether Aristotle quite manages to make the magnanimous as unslavish and unvulgar as he intends. In more Stoic accounts, which begin to dominate, this problem is removed, and it is removed in great measure by thinking of magnanimity less along the lines of justice and more along the lines of fortitude or endurance. The major such account, of course, is Cicero's in De Officiis (1.18-1.26).
In Cicero's account, the aspect of magnanimity which involves looking down on fortune is intensified. The magnanimous do not care about fortune. They are contemptuous of the goods of fortune, and equally contemptuous of its evils. They see things in perspective, and because of this, magnanimity has two elements: refusal to do anything except what is appropriate to important ends and actively seeking to overcome challenges whose overcoming would be of great value. Aristotle's magnanimity is largely a matter of self-knowledge; but Cicero's is an extremely active virtue, the attempt to live a life of truly great deeds done in a truly great way. And this leads to two interesting consequences for Cicero's account of magnanimity. First, Cicero's account makes magnanimity a much more philosophical virtue; the magnanimous philosophize. Indeed, in the Tusculan Disputations, one of his primary examples of magnanimity is Socrates. This element is nearly entirely absent from Aristotle's account, to such an extent that it sometimes seems from Aristotle's account that no one interested in philosophy could be magnanimous. In Cicero, on the other hand, no one not interested in philosophy can have greatness of spirit, because one needs philosophy at least to the extent of examining one's life and critiquing one's projects and pursuits in light of what is truly important. Aristotle starts his account with the magnanimous knowing their own greatness. But Cicero starts his with the magnanimous making themselves great, and that requires doing the rational work of determining what is truly, not just apparently, great. The second consequence is that Cicero's account makes magnanimity a much more public-spirited virtue. Because the magnanimous only devote their lives to what is truly important, they always place public good above private good, and this part and parcel with their disdain for a life in pursuit of wealth or fame.
It is unsurprising, I think, that Cicero's account becomes the dominant account; it has many advantages over Aristotle's. When medieval philosophers and theologians consider magnanimity, the result will inevitably end up being more like Cicero's than Aristotle's. It is almost impossible to imagine a Catholic saint being like Aristotle's magnanimous man, but it is not so difficult to imagine him or her being like Cicero's magnanimous man. Indeed, one of the most influential books on medieval Catholic thought, Boethius's Consolation of Philosophy, can be read as a treatise on magnanimity in the face of death, and it is a very Ciceronian picture. Even as Aristotelian thinker as Aquinas takes the virtue in a Ciceronian direction. Aquinas's magnanimous person (ST 2-2.129) is, like Aristotle's, concerned with honors for his or her own virtue, but this is understood, as in Cicero, as being primarily a work of fortitude, thus putting the emphasis on magnanimity being less about knowledge of one's own greatness and more about making oneself the sort of person worthy of great honor. Once Christianity enters the scene, the virtue of humility ends up taking a very important role in the overall sense of what is virtuous and what is not, and Aquinas has to adapt the virtue so that it recognizes this somewhat ironic superiority of the virtue of humility over the virtue of magnanimity. This requires Ciceronian elements.
In the modern era one finds a rather remarkable disappearance of the virtue of magnanimity, at least from serious discussion, with very few exceptions. One of the important exceptions is that Cicero of eighteenth-century Scotland, David Hume. We know from letters that Hume thought the emphasis on humility that laced his Calvinist upbringing absurd at best, so it is perhaps unsurprising that he explicitly attempts to bring back magnanimity as an important virtue (Treatise 3.3.2):
But though an over-weaning conceit of our own merit be vicious and disagreeable, nothing can be more laudable, than to have a value for ourselves, where we really have qualities that are valuable. The utility and advantage of any quality to ourselves is a source of virtue, as well as its agreeableness to others; and it is certain, that nothing is more useful to us in the conduct of life, than a due degree of pride, which makes us sensible of our own merit, and gives us a confidence and assurance in all our projects and enterprizes. Whatever capacity any one may be endowed with, it is entirely useless to him, if he be not acquainted with it, and form not designs suitable to it. It is requisite on all occasions to know our own force; and were it allowable to err on either side, it would be more advantageous to over-rate our merit, than to form ideas of it, below its just standard. Fortune commonly favours the bold and enterprizing; and nothing inspires us with more boldness than a good opinion of ourselves.
Given Hume's debts to Cicero, it's unsurprising that, despite adaptations to fit Hume's account of virtue, the Humean account of 'greatness of mind' is largely Ciceronian. While a good opinion of oneself has its place, what makes it a virtue is its boldness and confidence. That said, Hume's virtue seems to lose some of the public-mindedness of Cicero's: it is utility for oneself and the respect of others that Hume emphasizes, whereas Cicero certainly regards utility for the republic as the true sign of the great soul. It is interesting to speculate that this difference might arise from a slightly stronger mix of Epicureanism in Hume's view of the world. Cicero's magnanimity involves a very Stoic disdain for things of the world; but the way in which Hume's great-minded person looks on fortune seems more closely related to Epicurean tranquillity. Regardless, serious discussions of magnanimity seem to fade out after Hume.