Power supply efficiency changes based on the percent load it is under and usually peaks at around 80% load. I higher rated power supply lightly loaded will use more electricity than one with a lower power rating at the same load.
Look at it as though each power supply requires some power to run before adding the power used by the other computer parts and a larger one will require more than a smaller one. This is by no means saying that the power used by the power supply is constant. It changes based on load but a larger power supply will for the most part use more power than a smaller one.
So if you don't plan on upgrading your system to the point of needing that much more power over sizing your power supply will be a waste of money up front and cost more in power bill in the long run.
That being said if you plan to overclock or add a video card in the future it pays to buy enough power supply for your future needs and always buy a quality power supply for long term reliability.
It really shouldn't use more power any psu worth a crap (he has a corsair so it is good) will be at least 80% efficient no matter what load is under it. So at it's worst the psu will pull roughly 20ish% more power from the wall than it is using. If you buy a shitty rosewill 1000watt and put a 300 watt load on it (if it doesn't blow up) then it will pull more power since it isn't 80+ certified but a 300 watt load on that corsair will pull the same as a 300 watt load on a 500 watt corsair. Maybe a couple watt difference but close enough to not matter.