Hmmmmmmm... Crazy idea here, but have you thought of dumping size differences completely? FF characters don't seem to have much of a problem hitting anything from a tiny fly to a huge Weapon, nor grappling or throwing them. I mean, it might not be realistic, but it is in line with FF, is it not?
We've made a couple of conscious decisions with this game to emulate the Final Fantasy flavor as much as possible without sacrificing good gameplay mechanics. At a certain point, you have to say "this is cool, but doing this thing would throw balance out the window", so we've had to do a couple of losses of flavor and feel to make sure that the game is solid. This isn't a better approach than saying, for instance, flavor trumps all and mechanics be damned, but it is the approach that we're taking.
Also note that your to-hit not succeeding doesn't mean that you didn't hit it - it could also mean that you simply did not affect it with your attack, as it may have been deflected by armor or something similar.
We have done the impossible, and that makes us mighty.
Former BleachITP player of the one and only Minuet.