OP-
Its upsetting to you because you had a personal experience that causes you a different reaction that probably many others experience. I don’t find the sign terribly funny, but its witty and in context for the bakery. Doesnt bother me. Nor do I think it is insensitive to overweight people (speaking as a person with more than a few lbs to lose).
I used to pass a dry cleaners that had a sign in the window that said “drop your pants here”. I always smiled at that one.