Usually at 2 size categories below, though. Interesting anyway; I could have sworn there was some critter that started with 1.
What we have so far in the homebrews is just the greater version, so I don't think we need growth until we do the lesser ones. I assume, like the barghest, the greater version shouldn't advance.
So maybe Improved Critical (slam), Improved Bull Rush, Iron Will, Lightning Reflexes, Power Attack?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.