All trolls have ranging bulge sizes, but the size tends to correlate with blood status.

Obviously, highbloods have the largest; females tend to be bigger than males. The condescensce is huge, followed by the GHB, and so on down the spectrum. As it gets lower (around vriska and terezi) the sizes tend to even out. Karkat has a large bulge for his spot on the spectrum, a fact that Dave enjoys.