For the size comparison, bigger doesn't always mean stronger. Gear 3 > Gear 4 in size, but Gear 4 > Gear 3 in power.
Gear 4 derives its strength primarily from
DF/Haki harmonization, and increasing the rubber's elasticity coefficient. That makes it recoil stronger. With normal Haki if KKG is like 10x stronger than KG, than KG is like 100x stronger than base Luffy.