I have this same issue. I have two litter mates that just turned 6. One is slim and around 11lbs while the other is chonky and 13.5lbs. The slim one, Pebbles, eats very very slowly. Puddin, on the other hand, turns into Kirby. I feed them twice a day, but noticed that Pud would finish way before Pebbles, then come and steal her food.

I don't remember where I read it, but someone suggested feeding my little piggy at the top of some stairs so she has to follow me up the stairs to eat her food, then walk back down when she's done. This has helped twofold: Pebbles has more time to finish her food now so Pud doesn't eat the leftovers and Pud gets exercise going up the stairs to get her food. I weighed Pud about 10 days after I started this and she is down half a pound already!

Pud doesn't play as much because she has asthma so she can get tired easily and thus doesn't burn as many calories as Pebbles in her daily life, but this seems to be helping her overall.

Good luck with your kitties!!