He was pretty skinny at that point. For chubby look to after his marriage to Katherine, who must have been a good cook.
Aren’t most German women good cooks?
Anyway, it’s satire! Like there were cartoons of young George Washington chopping down the cherry tree, only the child had the middle aged Gilbert Stuart face as seen on the dollar bill.
Drawings of Luther depict a young man. Portraits from life show him older & tubbier (just like me!).
;^)