A type of caricature style of drawing the human male figure as having a really, really huge torso and arms, but really small legs. The latter is important, as simply having a big torso isn't enough. The effect the artist is going for here is almost a walking triangle.
When it involves superheroes, it is basically an exaggerated form of Heroic Build.
See also Lantern Jaw of Justice. Compare to Most Common Superpower, another kind of "top heavy"