Page 238 in the Core Rule book.
Quote:
The weapons presented in this chapter are of average size. But there are times where the heroes might face larger creatures wielding massive weapons. Weapons gain a Die Bump to their base weapon damage for each category above Average, without increasing the weapon's Speed. Unlike a heroes die bump which caps at d12+2, weapons which deal 1d12 deal 2d6 instead which each die increasing by one additional size category per die bump. Thus a Huge Scythe, Speed 7, d12 weapon would deal 2d10 (Mi) as a Speed 7 attack.
Any creature can, foolishly, use a weapon one size category larger than they are, but doing so imparts a -1 penalty to hit and increases the Speed of the attack by 2.
Bolding mine