Why did Willy punch F.H.Stewarts?

Stewarts alluded to the fact that Willy was fat, or so Willy thought from the mention of a walrus. Willy believed he was making fun of him (laughing at him).

"I’m fat. I’m very — foolish to look at, Linda. I didn’t tell you, but Christmas time I happened to be calling on F. H Stewarts, and a salesman I know, as I was going in to see the buyer I heard him say something about — walrus. And I — I cracked him right across the face. I won’t take that. I simply will not take that. But they do laugh at me. I know that."