You can make yourself a little more vulnerable to larger creatures in order to land a devastating blow.
Benefit: You can choose to take a –1 penalty to AC to gain a +2 bonus on melee damage rolls against creatures two or more size categories larger than you. When your base attack bonus reaches +4 and every four levels thereafter, the damage increases by 2. The bonus damage is multiplied in the case of a critical hit. You can only choose to use this feat when you declare that you are making an attack action or a full-attack action with a melee weapon. The effects last until your next turn.