I think the numbers of people in the parks are actually a lot higher than what the parks are telling you. I'm guessing that there can be several hundred thousand at a Disney park in a single day.
Take for example Cedar Point, who claims to have only 3,000,000 visitors a year. They have several major coasters, each with a very high capacity of riders at over 1,000 per hour. If 10 of their coasters have a 2 hour wait at one time, that is 20,000 people just at those rides. That does not include all the people who are walking, shopping, eating, or sight seeing, which is usually a much larger number than those who are already waiting in line.
So, take the Cedar Point crowds and double them for Disney.
But, Disney is way better at hiding their crowds than, Cedar Point. A lot of the major attractions have pre-shows, and people are constantly being moved around from place to place. They are experts at crowd control.
I wouldn't be surprised if there were a million people at the Disney World parks in a single day. But, they are not going to tell you that. I have been there, and trust me, they are there.