I mean, isn't the reason everyone draws Martin as fat because that's how he's described in canon? TMA is pretty light on character descriptors so when Martin describes himself as "not exactly the smallest guy in the world" it stands out as noticable.

? he could just be a little tall. like he just describes that it’s kinda hard for him to fit through a little window, which doesn’t really describe anything other than the way he sees himself. sort of just shows he’s an adult man