Why should they?
As pointed out, that gives unfair advantage to certain members. The citizens of others are poorly represented as the result, and deserve better.
"Members" are just abstractions. It's the people that count.
In any case, as already mentioned, Bajor in "Rapture" was said to have to choose representatives to the Federation Council, plural. It could definitely do with just one if there was only one vote. (And no, it wasn't a reference to Bajor having to choose a succession of people to the single position of Bajoran FCM - Admiral Whatley spoke of what has to be done at the point of entry specifically.) So we have a good reason to believe that members get more than one representative and more than one vote, although we also have a good reason ("Journey to Babel") to believe that members typically cast uniform votes. And we have no idea how the number of Council Members / votes is determined, although it would stand to reason that this is somehow related to the total population represented by the member.