Collars do not expand, but puppies and kittens grow quickly! If not loosened, collars can be deadly and literally grow right into your pet’s neck—an excruciating, constant pain. But too often owners don’t realize how fast their pets are growing, especially the larger dog breeds. So please check your pets’ collars at least every week until they’re full grown (that can be more than a year for the really large breeds of dog). You should be able to easily slip two or three fingers between their collar and their neck.
However, don’t let this simple task stop you from putting a collar on your young pet, since youngsters so easily get lost in their desire to explore the new world. And getting lost without ID is terribly dangerous and frightening to you both. So keep their collars on (with current ID tags), just not too tight.