Size Modifiers

It's an odd thing that there are a couple of rules in d20 that make no sense logically, yet work rather well in implementation. For instance, there's the whole "armour makes you harder to hit" thing that keeps getting bounced around. However, the one that's been really bugging me for the past eight hours are size modifiers to AC and attack rolls.

The principle of the size modifier to AC is really simple, and doesn't need much justification: big things are easier to hit.

The difficulty comes with the size modifier to attack rolls, which is a bit more confusing. Why should a small character naturally be better at hitting things that a big one.

The reason is a pure rules dodge - if a small character is fighting a small character, neither should have any bonus or penalty due to size. However, it's easier to give a permanent +1 AC than it is to recalculate AC all the time (how often do such modifiers get missed?), so small characters are given a +1 to attack rolls as well. Thus, although both characters get a bonus, these cancel out, and any problem goes away.

It is also a consequence of this that medium characters don't need to apply an AC penalty when fighting small characters - the small characters get a +1 to hit which has the same mathematical effects.

The problem I've hit occurs where a character is riding in a mech. Since mecha give AC bonuses, it rapidly becomes almost impossible for a Medium character in a Colossal mech to hit the broad-side of a barn (assuming said barn is made out of refractalloy or the like). Add in NPC class-based defense bonuses, mecha ace class features, and dexterity bonuses (would any mecha pilot ever NOT have maxed out dexterity?), and it becomes ridiculous. Colossal mecha have the ability to single-handedly dominate the battlefield? Please. The damn things are useful for precisely one thing - to stand there and be shot at.

Still, we'll see what my players think, when their carefully maxed attack rolls change from an average of +11 to an average of +2. The enemy AC values will remain at a nice comfortably 24 or so, of course.