Well, while we're on the subject, I don't know if any of you have read Scorecasting by Tobias Moskowitz and Jon Wertheim (great book and you're guaranteed to like it if you've ever read and enjoyed Freakonomics or Soccernomics), but they cover home advantage in depth using data on pretty much every major league. The real book does a much better job of going through it but this blog is a decent summary:
https://blogs.thegospelcoalition.org/kevindeyoung/2014/03/11/how-do-you-explain-home-field-advantage/
Basically they argue that over the long run, the most significant factor in home advantage is the referee (well, the referees being influenced by the home crowd). Travel and weather are shown to be largely irrelevant, and the idea that support from the home fans motivates players to play better is also negligible (though it's hard to measure this). And in their data, MLS has the highest home advantage (69.1% home team wins from 2002-2009) not just of other soccer leagues, but of all leagues they looked at (higher than in hockey, american football, basketball, baseball, rugby, and cricket).
Here's a link to the page of the book in preview on Google Books. You can note that the sport of soccer has a higher average of home advantage than other sports, as the referee in soccer can influence the outcome of a game much more than others.
It's hard to let go of the feeling that travel plays a large role in home advantage in MLS, but I can definitely see how MLS would have the most skewed home advantage while the data points to referees accounting for most of that advantage, since MLS refs are pretty bad.