The easiest fix for balancing size modifier is introducing wounding modifiers for relative size. The simplist is just downgrading piercing sizes based on relative SM, though that won't do much good at low tech since there's not much that does piercing. A more complete version, such as
mine, runs the risk of reversing the problem and making SM too
good (for example, under my proposal an Ellylon will be making death checks for almost any injury caused by a SM+0 foe).