I'm not sure Medinah really played much easier. Over twice as many players took part there. That gives a lot more opportunities to go low. e.g. Hideki shot two 63s at Medinah, but entered the event ranked 33. He wouldn't even have played, had the starting field been limited to 30.
At both courses, the median finishing score was around 280. 40% of all players shot under 280 at Medinah, and 40% of all players shot under 280 at East Lake.
At Medinah 3 players finished under 270. At EL, two did. But then Medinah had way more players. That suggests Medinah, in a way, played harder: fewer ultra-low scores compared to the number of players.
btw, according to ESPN the slope at East Lake is 137 from the tips. No matter what the course rating, that's real hard for me to imagine. Holes like 8 and 15 would take a horrible toll on bogey golfers. 9, 11 and 14 might too.