Class and Level Geekery VIII (spoilers ahoy!)

[Emanick]

An unadvanced Giant Stag Beetle is a large creature. A large creature should not be able to carry four or more medium creatures, which is just what we see them doing. In order to advance to huge size, they need extra hit dice, which increases their CR.

Good observation. But fighting monsters with a higher CR does absolutely nothing to give Roy less XP, which is the only way we can justify him not being 13+.