Based on a sample of effective college / university teachers in the late 1990 Ken Bain tried to identify the specific approach to teaching and characteristics of successful teachers (hence there is no systematic control group). This is not necessarily the professors with the best teaching evaluations but rather those teachers with students who learn, understand, and succeed.
The bad news is: There is no secret trick. It’s not the flipped / inverted class room, it’s not the use of fully animated power points, it’s not the extensive use of videos in class, it’s not overly generous grading practices, and it’s not paying students for their participation in class and doing their assignments.
Still, there was a common trait. It’s their approach to teaching that rather focusses on the who and not on the what. In brief, it’s student centered teaching and the teachers’ attitude towards their students.
I believe smaller classes (and a lower teaching load) facilitate developing this trait, though it is obviously true that also large classes benefit from such an attitude and the resulting approach. I am not quite there yet.