The total weight of your Main Weapon + Armor + Sub Weapon is your 'Battle Weight'. Your Strength and Talents determine your threshold; exceed that threshold, and for every 1 point you exceed it by, you suffer -2 Evade and -2 Hit. (You do not receive any benefits for going under.)
Some materials, namely Galdric and Planetarium (which reduces weight, and the latter reduces by a little but grants some weapon statistics bonus), and certain enchantments (namely Floating), can dramatically reduce your Battle Weight.
Your Battle Weight is displayed below your HP and FP; the number on the left is your current Battle Weight, and the number on the right is your threshold. So 5/12 BW would mean you were under your maximum before you began taking penalties. While 24/20 would mean you are 4 over, meaning you suffer -8 Hit and -8 Evade.
| Damage vs. Protection|
Hit vs. Evade
Critical vs. Critical Evade
Status Infliction vs. Status Resistance