If you try dropping one villager on another of the opposite sex, do you get a message that you're at the limit? If so, that sounds like a possible _bug_, provided you're entitled to a higher population through getting full sets of collectibles.
My population is at 90 right now. I have two collections complete (still missing 2 beetles and three pebbles to complete the others) and all puzzles complete, which means I should have a limit of at least 100, if not a bit more. When I drop a male on a female, or vise-versa, I get the pop up saying that I've reached my population limit.