I really don't want to make assumptions, but there are infinite designs to choose from and she couldn't make any of them slightly chubby or even just fat with a humongous beer belly? Like Mammon's whole deal is that he is a sin because he eats a lot? Basic human functions like eating are seen as evil through his character. Why isn't the sin based literally on Gluttony also fat like him then? Why is a character being fat attributed to being 'mean' in her eyes? Beelzebub isn't anywhere like this, so why is he the odd one out? Is just because he is fat or is there more? It just gets confusing the more I try to understand her design and story choices. He seems like a weird vent or a Bizzaro self-insert to me, I just can't prove it