Yes, I guess you can simplify it like that, just to get the gist. Larger will not only get you efficiency, but extension as well. I suppose that could be important, come to think of it, since your sub is going to be far from the speakers, which necessitates a low xover.
I'm not going to go into the reasons right now why I chose PSB Image T55s over T45s for the HT, but my primary complaint is bloated midbass. Now, there are people who think the T45s are better in this regard. I have no idea how much of it is the cabinet vs other things, if this is true.
I remember the knowledgeable no.5 once mentioning that the smaller KEF IQ5 had better dispersion than the larger IQ7.
Anyways, I wouldn't use driver size as an important selecting factor. If you could indeed narrow it down to two models in the same lineup, the first thing I might do is bee-line to owner's forums and ask the owners, and salesmen, this question directly.
Are the xover points identical otherwise between those Studio bookshelves? How does that affect the overall performance? How does different port tuning choices affect the overall performance? Don't tell
me the answers, because I won't be able to help you.