I adopted 2 kittens at the same time. One was skinny. I tried free feeding. The skinny one didn't gain any weight and the other cat turned into a round blimp.
My blimpy cat has to have meals. He would be severely obese if left to his own devices.
I use marbles on his food. They are the right size that he can't swallow them and they slow him down. Only they are a pain to wash and clean!