K kenco I got to have a playtest with a monster with +vs size differences, and it is already pretty brutal as it turns out! It definitely didn't feel like it needed damage bonuses, because it could ignore damage so well. When it rolled poorly it didn't lose many dice, but when it rolled well it inflicted lots of damage anyway.
Also I might make Size ignore only damage, not damage and facts. It was intended to stop you knocking a human over with a single success, but it actually just felt tactically boring.