So there are two groups, distinguishable by physical or behavioral traits, and each group lives in communities that largely excludes the other. So each takes itself as the norm — the unmarked case — and views the other as marked (special, notable), and consequently has a special term to name the other: roundhead, pinhead.
It is the way of such things that these terms quickly become seen as slurs, even if they weren’t meant that way originally.
And then there’s Herbie Popnecker, the fat superhero (Wikipedia entry here); of course, Bill Griffith wouldn’t make something like that up. Here’s Herbie confronting another preposterous comic-book superhero, the Flaming Carrot (Wikipedia entry here):
How phallic is that?