Most of the 100,000+ person stadiums arent actually larger than NFL stadiums. The trick is they use bleachers which take up less space per seat, so a smaller stadium is able to fit a lot more people. I went to Michigan and now have season tickets to the Chicago bears, the big house fits 45 thousand more people, but the bears stadium feels much larger. I think everyone being so close together adds even more energy to college games.
Pro stadiums are often bound by capacity limits from their cities agreement with them being there. So over time they tend to replace cheap seats with more luxurious experiences and booths. Dodgers stadium is kind of notorious for this.
