User blog comment:Mckrongs/Official Nintendo Magazine talks about Monster Hunter 3 G Again/@comment-4980054-20120402054054/@comment-1726879-20120419115913

And when he says small, he doesn't mean killing giggi all day long, he means a small monster of said species, such as a small rathalos. It's pretty random, but I believe that high-rank missions had the possibility of even smaller/larger enemies when compared to low-rank. Don't quote me on that though.