If you look at the respective number of applicants for say UCLA and UCB vs USC and Stanford it seems that there are factors that work to increase the number of people who apply (attractiveness of location, prestige, etc.) and factors that decrease it (particularly that it’s not worth applying because students think they won’t get in/can’t afford it). I assume that the relative cost is an element of the gap in the number of applicants between the public and private universities (and selectivity an even bigger factor for Stanford). However, part of the gap between UCLA and UCB may be because some students feel that they won’t get into Berkeley whereas it’s worth giving UCLA a shot (though Westwood may also be more attractive as a location and has a larger SoCal population to draw from for those who want to stay local).
So although there’s a general increase in the number of applicants across the board, I’ll be interested to see if UCLA’s increasing selectiveness means that in the next few years more students will conclude its not worth applying because they don’t think they will get in, causing the gap in total applications compared to Berkeley to narrow somewhat.