This can roughly be seen by the difference in voter participation in swing states vs non swing states, not everyone who doesn't vote at all. (I'm assuming the people who don't vote in swing states wouldn't vote under a popular vote or different election system).
There's also the fact that get out the vote campaigns and campaign advertising would be conducted differently under different election systems. Under the current system, neither presidential campaign advertised much in Vermont or Oklahoma, but under a different system they might have, and the actual percentages coming out of those states might have been different.
All I'm really trying to say is that it's difficult to look at the results of one election system and easily predict what the results would be under a different system. The results under our current system may still be generally representative of what we'd get.