Logically that's true. But musicians are all too often not much concerned with money. Getting popular tends to trump everything else. Also, the concert stages are often so small, especially when you're starting out, that "just fitting everyone on stage" becomes a concern, too.
However, if you look at successful power trios, for instance (Cream, ZZ Top, Grand Funk Railroad), I think the original three decided that sticking with "power trio" as the sound they wanted was the reason, rather than "Let's not add another guy to split the money with!"
If you add another instrument, then that changes all your arrangements, unless they're just doubling someone else. More to the point, the group dynamics would be drastically different: you all have to agree on things.
You'd have to decide that the additional sound is worth all the hassle. I'm thinking now of Sufjan Stevens, who has a whole bunch of unusual instruments in his band, and he definitely decided the additional sounds he could get were worth it (I don't know if they all get salaries or a cut, though).
You see the phenomenon in songwriting disputes. Robbie Robertson claimed all the songwriting credit for The Band, and thus he gets all the money. Modern pop stars often make gratuitous changes to the songs the producers hand them, so they can share in the royalties ("change a word, get a third").
So I guess the answer is "Yeah, sometimes, but not that often a primary motivation."