They seem to fit them well but as long as they are indoors they don't need a collar. Also you could improve how they feel about their collars by giving them treats while they wear the collar so that they have a positive association with it.
This was Charlie at 3 months. The thing that looks like a tassel is actually the result of his incessantly licking it. One night, he finally got the collar off, and the state of the tassel made it impossible to put it back on. I gave up on a collar after that, and had him chipped when he was neutered.
He used to somehow figure out how to get the latch in his mouth, then he'd crush the latch. I've had to throw away several collars because of this. NOW, he seems okay with a collar. And that's good because he's MUCH calmer when he has collar on. (have no idea why).