I’ve put some thought into this, and aside from the technical implementation (which is above my pay grade) I think the Qubes Domains widget should show the amount of RAM the qubes are actually using, as in not including anything that would be taken if needed. As an end user, it is confusing to see that a “3894MB” next to qubes and not assume that is how much they are using.
On the technical end, maybe it may be a good UX idea to balance the qubes such that they inly have what they need instead of taking it when something else needs it? I agree with @Euwiiwueir—I prefer to keep the usage down to understand what my machine is doing, and I think the current setup is a little confusing for a user trying to manage his resources. I’m not advocating for changes, just adding my opinion.