Her shoulders look broad, but they’re actually not. Her little coat sort of has padding, like shoulder pads in some suits women wear. Boy characters often don’t have shoulders (or you know, anything that would make them stand out like padding on a uniform), so because Meli is always hanging out with Milo and Zack, and not around other girls, it’s easy to say she has the broadest shoulders.
Sometimes the animators will follow the character constructions to a tee and make sure to show the difference between Meli with the coat and Meli without, and sometimes they won’t get it right. But humans aren’t perfect, and whether her shoulders show as broad or not when wearing the coat is not a pressing matter lol. Not unless the show was focused on her shoulders XP
tl;dr: Compared to other boys, Melissa has the broadest shoulders. But she has shoulders as wide as any other girl on the show her age; the coat she wears is deceiving XD