Regarding average class size, note that the average class size is not the same as the average class size of a given student, because the large classes have more students in them.
For example, suppose a college with 100 students has each student take two courses. One has all 100 students, while the other us taught in 10 sections of 10 students each.
Average class size is (100 + 10 * 10) / 11 = 18.2 (rounded). But each student has an average class size of (100 + 10) / 2 = 55.
For percentage of classes under 20 students, that can be misleading in a similar way, since fewer students take those small classes. But also, “classes” may include discussion or recitation sections attached to a big lecture.