To me
Disneyland almost always has more crowds. I have been to WDW during Christmas week and it was nothing compared to some Saturday's I have had at Disneyland. WDW has a lot more room/parks/resorts for people to spread around in and that helps.
As others have stated, when Fantasmic is running at DL, that makes getting around the park a logistical nightmare. I have never seen anything that can touch the amount of people that cram into the Rivers of America area at DL when I have been in WDW.
Fireworks viewing in MK is also way different. At DL, you can pretty much forget getting a spot in the hub near showtime. At MK, last weekend, we saw Spectro in Frontierland and then were able to get to the hub without a problem and get a good spot with room to spare about ten minutes before Wishes started. That would never happen at DL, especially on a weekend.
So to me, crowds at WDW can and do happen and in a big way. But the human traffic jams that I have come to expect at DL have never happened to me at WDW. In the peak of summer, holidays and all the other times I have been. The sheer size of the parks at WDW, Epcot in particular can hold a lot of people. Wait times are comparable for rides, but getting around, finding spots to stand for a show or parade, are all very much easier at WDW based on my experience.
And obviously, I am talking about Disneyland itself, not including DCA which is almost always immune to this sort of thing.
I think anyone who has learned how to navigate their way around DL will be surprised at how much easier it is at WDW within the pakrs themselves. Now getting from park to park is a whole different thing. And at night, at park closing (MK in particular) its amazing how when all those people spill out how the ques for busses, boats and the monrail get packed. But in all, that compactness that makes Disneyland famous for human traffic jams isn't there at WDW.