Full grown

How old is a cat when its full grown.Because my kittens are 6 months old and they look little still.

LOL, they actually come in all sizes, but on average a female usually hits about 9lbs and males about 12lbs. They are still very young at 6 months and will probably keep growing until they are a year to a year and a half. Unless of course they have Maine Coon way back in there and it pops up...they may grow until 2 to 3 years.

Some of my female foster cats are very small, and don't seem to be getting any bigger, even though they're young. Does getting pregnant shortly after the first heat and having kittens stunt a cat's growth?

Females are usually a little smaller than males on the average. Yes, when a cat gets pregnant at a young age, it tends to stunt the growth. Only because all the nutrition and hormones are interupted to keep the babies alive. If you look at a healthy cat over a year old that has kittens, you can tell that everything they have is going to take care of the lil ones.