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.
I suggest buying the cats a cat jumper for times when it gets particularly cold to keep them warm, there are many to choose from on Amazon, hope this helps!
I suggest buying the cats a cat jumper for times when it gets particularly cold,it would be beneficial by keeping them warm, the cats would be very grateful and it would look cute at the same time!