Calves only get so big. . .depending on the body type you mean.
Some guys have big calves, some don't. I have pretty big calves (all muscle) because I pitched, played center field, ran track (just for one season though), and played both basketball and football too in middle school and high school. Guys that just wrestle don't tend to HAVE to have strong calves like all-sport athletes or athletes of specific sports (like soccer) since wrestling doesn't require running around and rapid changes of direction a lot. Nor does MMA, which is why those guys also don't have very big calves.
Though as far as Brock goes, I don't think it matters since his quads and upper body are tank-like. I am a little surprised they're not, say, 10-15% bigger? But if you compare and contrast his calves to say. . .CM Punk's whose calves look like little twigs that could snap at any moment any time he lifts someone up for a GTS, then Brock's don't exactly seem small by comparison. They just seem slightly undersized in comparison to the rest of his massive body.