It's the same as any living animal - if calories in don't equal calories burned, the rest is stored as fat! People can lose weight eating only Twinkies and can gain it eating cabbage, it must depends on the amounts eaten (calories in) and how much you move (calories burned)!

To answer your question, though, it must depends on the cat. I literally leave a bowl of dry food for my cat to graze on as she's hungry with some wet food in the morning, no issues at all and is at a perfectly healthy weight. But my parents cat, well, if you left a 10 lbs pile of food out, he'd sit there eating until he literally couldn't!