A cat's size can change for many reasons. If your cat has always been thin, it may be because of his genetics, or it may be because his diet and exercise are insufficient. However, if your cat has recently started to lose weight, it may be due to an ongoing illness.
If your cat has always been very thin, but its appetite is normal, it may be because its metabolic rate is high. This means that its body will use up energy quickly, even if it eats a small amount. It is recommended that you take your cat to the vet to determine if it has any health issues. Your doctor can give you more specific advice to help your cat maintain a healthy weight.
