Why does class size matter?

I grew up in a school district with a class size ranging from about 15-20 kids, grades K through 12. Naturally, the classes grew a little bit as I got to high school due to gym classes being larger and just more students moving into my area as the years went on. As I moved on to college, my class sizes stayed the same for the first 2 years at 15-20, and this past year I noticed a change. My classes within my major (Communications at Widener University), began to shrink to about 10-15 students per class and I noticed one side effect of a small class. Soon enough, there was a greater overall closeness with my peers and teachers. It gave us a feel that we were in it together and we all genuinely wanted each other to do well.