If I’m not mistaken, GFist ignores DEF regardless the monster’s Size. The damage output is proportional to ATK and the current SP the player has and additional damage from Cards. For example:
I used Lv5 GFist with 286 ATK(Weapon Atk+Base Atk) and 679 current SP I will expect my output damage of 22,707. (using the formula: ATK+(8+SP/10)+ Skill level Bonus damage).
As far as I know, weapon perfection doesn’t really affect the skill. Though its up to you if you feel something’s off with the calculation.