I mean, Bruce is a BIG GUY, and gives the impression of being bigger much of the time. Dick has never been presented as particularly tall, not helped by hanging neat Bats and Kory all the time. And he has an acrobat's physique while Batman is more of an all-around bruiser. Batman can physically scare you just with his size. Dick's more about confusing you, tricking you, going around you, etc. Batman stands alongside Superman all the time and seems the same height. That's big.

And for those of you I know focus on such things, god help Dick if anyone ever got a good look at both asses. ;)