I think you have to be careful here just adding those numbers like you did. Let's take a Regular Greatsword vs a Greatsword that you need to be size +3 to wield.
Init Atk Dfn Dam Str Load
Regular +2 +5 +2 +9 +1 2
Size +3 +5 +8 +5/2 +12 +4 5
If we assume that each creature has the required strength to wield the weapon and their score in 2hd weapons is the same. I am also assuming Dex and Quik are the same, for the sake of demonstration.
Initiative - probably no worries here since the weapon is larger/longer---they should attack first.
Atk - It is a bigger weapon, so maybe no worries.
Dfn - The weapon is larger, but it is covering a larger being (since you need to be +3 to wield it) I would leave it at +2.
Dam - I am increasing the damage it does, since the weapon is bigger, which makes sence.
Str and Load - Both these are fine
Now, given these numbers, the Size +3 weapon will:
Attack earlier. (+3 to init)
Hit more often. (+3 attack)
Be hit the same. (+2 Dfn)
Do more damage. (+3 attack and +3 damage and +3 str (or more))
So that's +9 more damage, which will seriously sway the battle.
Now this can be compensated for by lowering the weapon skill, Qui and Dex of the mob, which in turn will:
Attack at the same time or later.
Hit at the same rate or less.
Be hit more often.
Still do more damage.
Thus, you need to tweek all these numbers so the fight comes out fairly. So, just as long as the mob wielding the weapon gets the added damage from having a high strength, things should be ok. Remember, these rules were written for fast and easy combat: not to have all situations fleshed out. It would be great for an official supplement to come out with revamped combat rules, but until that happens, I think you need to stick with what we have so you don't unbalance the fights.