So you can wield an oversized heavy mace (albeit with a -2 penalty), and deal the same damage you would with a greataxe - 1d12.
I think the MWP feat is worth it, if only for eliminating the penalty. But anyway -
If you actually had the Martial Weapon Proficiency feat, you could wield an oversized greataxe and deal 2d8 damage!
Seems balanced to me.
It is worth noting that wielding oversized weapons is almost never worth it - you can either wield a medium greatsword at a -2 from power attack, and deal 2d6+4 damage (average 11), or use a large greatsword with no power attack, but with a -2 penalty, and deal 2d8 (average 9).