I know it's probably more of a function of the actual walking areas, but I always feel that Disneyland is more congested than the Magic Kingdom on comparable days. With that said, Disneyland could use a few attractions to boost capacity as well.
I also wouldn't be surprised if the Fantasyland Expansion does result in a permanent attendance boost in the neighborhood of 750K-1 million, presumably by 2015. The 2010 TEA report isn't out yet, but in 2009 MK had an attendance of 17,233,000. Considering normal growth that has been happening recently, I don't think 18 - 19 million is unreasonable for 2015, the first full year the expansion should be complete.