He kind of looks like my Rocky- the heavy side of normal. Your cat does not look like he is dangerously overweight, and like others said a vet would be the best to consult- some cats are just bigger boned.
It's especially hard when you have more than one. Rocky was sneaking the kitten food! I have decided to start giving them adult food b/c he kept eating it and they kept eating the "grownup" food anyway! BTW they are 8 months.