Endowments matter, but it is usually best to talk about endowment per student and not necessarily the overall number. Of the elite schools this cohort adores, St. Paul’s is second only to Exeter these days. Groton has a very high endowment considering its size, comparable to Andover. Deerfield, the upstart, late to coeducation, is playing in the same sandbox as Andover and Groton. Hotchkiss is right behind. Choate has a very much lower endowment, about half of Hotchkiss.
These numbers fluctuate somewhat, but the hierarchy is not likely to change much and has not in fact change in forever, except for Deerfield–Deerfield’s really come up in the world since they returned to coeducation and have become a true peer of those other schools, thanks in part to all that cash.
That said there are indeed other schools with high endowments, including St. Andrew’s in Delaware, Peddie, and Mercersburg. Lawrenceville’s endowment is very close to Choate’s, as is the size of the student body.
What the long-standing high endowments get you are those luxuries and sundries. Like financial aid. That’s a good one. Choate’s FA policy is not even on the same planet as its peers. Plus things every school needs like the new boathouse with a huge lounge or the spiffy student center with a dozen 9-foot pool tables or state of the art dorms or the second hockey rink or pool with diving well or a dedicated football practice field or whatever. Nobody NEEDS any of that, but if the other folks have one or more of those, well then you do kinda need those things. Or a billion other things.
The schools without the sky-high endowments are not likely to be short on flasks in the chem lab or chairs in the dining hall or basketballs in the gym, but don’t think it doesn’t matter. It does. There is an arms race of science buildings and boathouses and stuff and the schools with the bigtime endowments have a much easier time keeping up without endangering the existence of the school or the ability to be as selective as they wish.
Look at what Northfield Mount Hermon did a few years ago: they cut the size of the student body in half. That made the school much healthier financially. A generation ago, there were at least four huge boarding schools in New England, with Andover, Exeter, Choate, and NMH all having over a thousand students. Only Andover and Exeter have been willing and able to maintain that student body size. Choate cut the size of the school in the mid-late 90s iirc, at least in part to make the endowment look better when compared to its peers.
Everybody should be willing to peel away the curtains as best they can when they consider these schools. Peek in the corners. Don’t be overawed by any one thing. Look closely and see if it looks like the kind of place for your family. Money isn’t everything, even at boarding schools, but more FA probably, hopefully means a more diverse student body, for one. But the smaller schools often pick and choose their top FA candidates to build their classes to the best of their abilities, so it is possible that one school will offer much FA for a particular applicant while another school offers none or simply rejects that same candidate. As always, make your lists well and hope to find that important good-fit school that loves your family, especially if you need FA.