It's rude to insult someone based on their size.
Tom is very skinny, and his friend, Devin, is very big compared to him.
Tom wants to talk to Devin about their weight.
Tom tells Devin that he wishes he were as big as Devin.
Tom tells Devin that he has to lose weight if he wants to keep hanging out with him.
Devin feels better about his own weight and likes that Tom doesn't judge him.
Devin's feelings are hurt, and he doesn't think Tom likes him that much.