Repeatedly in a grapple, you need to make opposed grapple checks against an opponent. A grapple check is like a melee attack roll. Your attack bonus on a grapple check is: Base attack bonus + Strength modifier + special size modifier

Special Size Modifier: The special size modifier for a grapple check is as follows: Colossal +16, Gargantuan +12, Huge +8, Large +4, Medium +0, Small –4, Tiny –8, Diminutive –12, Fine –16. Use this number in place of the normal size modifier you use when making an attack roll.