When I first went up agains Mars' Purugly in D/P, I was baffled on how a 100+ lb. Pokémon could have base 112 Speed. But still, I think Speed in battle and speed outside of battle are different things (in the games at least).
But still, that didn't convince me to put Purugly on my teams. Scolipede is bigger than Purugly and Dragonite and it's faster than the latter and just as fast as the former.