Actually, it is the opposite. The drives are all the same size in millimetres, but with the higher capacity drive, the data would be more concentrated in one area. If you have 110 GB used on a 120 GB drive, the drive head has to go all the way from the outside of the drive to the inside to access all the data. Same 110 GB on a 320 GB drive, and only a third is used, so the drive head can stay within the same third of the drive all the time. That's one reason why a bigger drive is faster if everything else is equal.
It could be that the new drive just uses more power; some drives are better, some are less good. Or that it stops more often when idle and needs to be restarted very often, costing extra power (cheaper to let the drive spin instead of stop/restart/stop/restart). Or it stops less often when idle, wasting energy when the Mac does nothing (cheaper to spin the drive down if it isn't used for ages).