I think if you break it down, there are about 100,000 people in the resorts on any given day. (30,000 rooms at 3.3 people per room). If you take that out for a year, that's 36 million or so people in the resorts for the year. Last year, estimates were that WDW had a total of 50 or so million visitors, so I think the percentage of resorts guests at the park is closer to 60 percent. That's why there are no slow days anymore, because the rooms have to be filled so they find a way with discounts or whatever.
If you look at days when the parking lots are empty, the busses and the parks still seem full. That's resort traffic, IMO, and I think it's the reason the crowds are so plentiful so often.