Kind of rare, but not exactly unheard of. Kinda like anyone taller than 6' 8" these days. If you saw them, you might think "Wow, they're tall" but you wouldnt think of them as freakish.

Combine that with this being a Roman fort, and the fact that being a soldier was probably a pretty common job for big/strong men, and it's really not that surprising. Kind of a nothing story really. Imagine a bunch of future archeologists scratching their heads over finding a bunch of large shoes in an NBA locker room.