Just as people can put on a little extra weight, so can cats eat too much and get obese. To keep your cat in shape, first identify whether your cat is obese or not. Some cat breeds are naturally larger than others, but overweight cats can come in all breeds.
To help a cat lose weight, offer it less food and make sure it exercises more (good advice for people too). Use a user toy or other activity daily to keep your cat moving, and be patient. Cats can get fat and lazy just like people so getting a cat back into shape can take time (just like people).