Reading several of the food posts on AIBU, a common thing I come across is people who say, usually about their DH or DP, is that oh my partner needs loads of food and it doesn't matter because he is slim/thin etc.
You can still be greedy and be slim. Stuffing loads of food isn't a good thing and it doesn't make it ok just because you don't visibly look obese.
Slim people can have high levels of internal fat so may not look fat