Generally it's more about the number of chips you need. If you can get to x GB of RAM with the same number of memory chips, just higher capacities, then the power difference is truly quite miniscule. If you have to double up chips to reach the capacity then you start drawing more power (though still on the order of a couple Watts max difference at those sizes). Even then it's not always a constant couple Watts, RAM+memory controllers uses less power when you're not actively writing data.