People disagree on how long a kitten should be on kitten food. In my experience I only feed kitten food for 4-5 months at the most, then they go on adult food.
I have never had a fat cat doing this and since most kittens are neutered young, the extra calories in kitten food can put on too much weight and make them fat. Only reason to be on kitten food for over 5 months is if the kitten has been sick or a poor beginning in life or its a larger breed like a Maine Coon (then keep them on kitten food till about 9/10 months old.
So if your kitten is doing fine, gaining weight and is healthy, I don't see the need for feeding kitten food at 6 months old.