I said in a previous post that I didn't think that Thomas Aquinas's esse overlapped very well with our 'existence' (unless we are deliberately using the latter in an unusual way, which does sometimes happen). There are lots of reasons for thinking this, actually; but you'll just have to trust me on the quantity. For this post I thought I'd just flesh out one of those reasons. It starts out with an analogy St. Thomas makes in the De Ente et Essentia:
Just as if someone through one quality could perform the operations of all the qualities, he would have every quality in that one quality, so God has every perfection in His very esse.
I find this an interesting analogy. 'Existence' as it is often used indicates what we might call a mere fact: existence is the bare fact that something exists. It is clear that this is not anywhere in the vicinity of what Thomas intends. He really does have an emphasis on the actual of 'actual being'. If something actually is, it is because it is 'in act', actual; and because it is actual, it is active. Esse is not quite an action -- what we generally think of as actions, of course, all presuppose actual being. But it is certainly not the bare fact that something exists -- it is that about the actual thing that makes it to exist, which makes it actual, and actions are possible because they presuppose a more fundamental act, namely, the act of being.
So we have an analogy here between activated qualities (like powers) and divine esse. It is only an analogy, of course, but the point is that the analogy can be made. This is why Thomas has no problem saying that God's esse or being is abundantly perfect. Just as a high-level quality can have the perfections or excellences of lesser qualities in a higher way (because it can, as he says, effect the operations of those qualities, and lacks many of the limitations of those lesser qualities), so can God's esse have the perfections or excellences of being that lesser esse's have, but in a more excellent way. Aquinas's esse or being is something for which it makes sense to talk about 'fullness of being' or 'richness of existence' (if you must translate it as existence). And this is all the more important when Thomas talks about God's simple being. This simplicity is not a mere-ness (as it were); to say that God has simple esse is not to say that God merely exists. Quite the contrary. Think of the analogy again. A more excellent power, when activated, can do more and be more than lesser powers, but it can do so in a more unified way -- what you can do with two more limited powers, you can do with one less limited power. That's crude, of course, but it gives the idea: the greater the excellence of the power, the more it can do, and the more it can do as one power. Analogously, the more excellent the being (esse), the more perfect it can be on its own. Unlike creaturely being, which requires all sorts of supplementation in order to be fully excellent, divine being needs no such supplementation; it's rich, abundant, overflowing with excellences, and can do more than creaturely being can.
The analogy is rough (Thomas doesn't intend it to be more than rough), and there is a lot more that can be said about this, whether for or against or in order to develop it further. But it makes clear a reason why there is more to the Aquinate notion of esse or actual being than we usually find in our usage of the rather spare and weak term, 'existence'.