Funnily enough the sprite based enemies were a real strength for Doom compared to many of the fully 3D games that came out in the next generation, including Quake. Because rendering full 3D enemies was taxing for the hardware you couldn't put too many of them on the screen at once. In some games more than 2 or 3 would cause it to chug. So you only ever run across one or two enemies at once in the games, unlike Doom that wasn't afraid to throw a swarm of pinkies at you because you just picked up the chainsaw.

Doesn't matter, to the Doom engine they are tall cubes , albeit with increasingly awful collision detection as the cubes increase in size.