The reason for a big power supply is the same as the reason for a big engine in a pickup truck.
A 4 cylinder engine will work perfectly well getting a Chevy Silverado from stoplight to stoplight. However put a 32' Scarab on a three axle trailer behind it and drive to the Keys and you got to have the horses.
Same with a power supply. Voltage fluctuations will happen on the lower watt ps's when the CPU is put under a load. Those fluctuations won't happen on a higher wattage ps.
BT
EDIT: Motherboards are not voltage regulators. They use what they need and no more. If they need more than the ps can supply then performance suffers.