The modifiers are static, and make attacks relative to each other appropriately adjusted for the situation. The equation bakes the advantage / disadvantage of size modifiers into base stats, and makes no impact on two creatures of the same size fighting each other.
These size modifiers are applied to a creature's attack bonus and AC.
Using the idea of equivalent equation, the size modifiers automatically take relative sizes into account.
For example - two tiny creatures duking it out are effectively at +0 size modifier against each other. +2 Hit, +2 AC VS +2 Hit +2 AC = total of zero modifier relative to each other. So they are on equal footing size wise.
A Colossal creature Vs a tiny creature
+2 Hit, +2 AC VS -8 Hit, -8 AC = +10 modifier in favor of the tiny creature. No need to re-apply the bonuses a second time.
A colossal creature VS a Colossal Creature
-8 hit, -8 AC VS -8 hit, -8 AC = total of zero modifier in relationship to each other.
A Gargantuan Creature fighting a tiny creature would be at a total + 6 modifier, and a Gargantuan creature fighting a medium creature would deal with a +4 modifier.
So it scales and auto scales. The math is already worked into the base stats of the creature to prevent excessive calculations during combat.