What does bullet weight, grain, have to do with light loads and heavy loads? Light loads referring to the kick, or a mild load. Take the 9mm 115 gr vs. 124 gr, standard pressure factory fmj. The 115 gr bullet is smaller, lighter, so do they use less powder and / or add filler, because wouldn't you have more room for powder since the bullet is smaller that say the 124 gr bullet?
Or take the .44 Rem. Magnum. It seems that the heavier bullets, 300 gr, are reported to have more kick, penetration, than the lighter loads.