sorry to sound dumb but, wouldn't a bigger PSU use more power?
No, a 800W PSU will give UP to 800W of power, won't always use 800W
Then there is efficiency to take care of consideration. A PSU at full load wastes more energy than another at less load
correct. I have an evga 1000w psu gold rated powering 4x7950 and it is pulling 1400 w from the socket