In comparison to other colleges further down in popular rankings, students accepted to ONE of HYP may not have as many “better” (by perception) choices to “jump off” to - and those who are on the waitlist, may not also be on other “competing” waitlists.
So while down-list colleges need to have big wait lists to fill their lower yield among the accepted, they might also have a much lower yield from the waitlist itself (because of people being on multiple waitlists), further driving up waitlist size.