My kitten seems to be getting fat, help!?

I own two kittens, about 4 & 5 months old. One appears to be chubby while the other one seems to be filling out (well at least she is compared to when we first got her-she was a stick). I'm not overfeeding them though, at least I don't think. They share a dish of cat food and we fill it once everyday. Is there a rule to how much dry food they should be eating a day? I feed them the purina brand kitten food.