^The other question is the freshman experience. At many universities, upperclassmen will have many choices of small classes, in their majors, etc. But kids who are attending a top LAC will have all or mostly all small classes, even as freshmen, whereas freshmen at larger universities may be in many lecture classes. That’s where the “only two percent” statistic @Publisher quoted comes into effect— in how many students are taking large lecture classes, especially freshman year.
A big exception is writing seminars or freshman seminars; many of the kids who are freshmen at large universities have these as well. So even larger schools ensure freshmen get to be part of a discussion-oriented group with close attention from a professor.
But even at Williams, you can have a large intro course. Intro Art History and Intro Psychology are lecture courses not so different from those someone might take at a large university! As @Publisher points out, does it matter if there are 75 kids or 400 kids in that lecture?
(As I recall, there are some collleges- I think Grinnell was one?- who do not have ANY large classes, but these colleges are few.)
In general, the difference in class sizes might be that, first semester freshman year, a kid at a top university might have three large lectures and one small class, and a kid at a LAC might have one large lecture and three small classes. And, at a LAC, it may be possible to have zero large classes, ever, depending on which courses you take.
(Disclaimer: I am sure there are many different experiences and many exceptions.)