Like the others said, you don't want a 'fat' cat because of potential health problems. If yours are tiny like, sickly and they need to gain weight, you could try feeding them kitten food which has more fat and protein.

Cats, like people, are going to have different body types. When I adopted mine, one was tall and lanky, and still is. The other is big and fluffy. If your cats are healthy now and are just built small, keep them that way. You can always adopt a big fluff ball later