Look beyond the surface: Thacher is tiny and can’t compete facilities wise, for example. But if you are interested in Astrophysics or coding the programs that run the Thacher observatory or crunch the data, you could find yourself part of the only high school group presenting papers at Astronomy conferences alongside university researchers.
But I do agree with @Happytimes2001 … If you are already spectacularly advanced and accomplished at anything (sports, STEM, arts) and you know you want to specialize, a large school will have more to offer. Just bear in mind that it may be harder to try other things you are intrigued by but not yet expert at since those classes, teams, and clubs are also more likely to also be saturated with experts who specialized early.
I happen to believe that most kids would benefit from more (not less) exploration in their teen years. It is a great time to take risks and try new things. Leave specialization for college. But for those kids who have a singular focus and are already quite advanced and accomplished, it can be quite exhilarating to no longer be the only one and actually find large group of similar peers at a larger boarding school.