A destinction should be made between magnet weight and magnetic assembly weight. The only weight that matters is that of the magnet it's self, and the marketing types usually quote the weight of the magnetic assembly, not just the weight of the magnet. Additionally, not all magnetic materials have equal magnetic flux density per unit of weight.

All things being equal, a larger and stronger magnet (with a correspondingly larger voice coil assembly) will have more motor strength than a small assembly - no surprise there.