Class differences

This semester I am teaching two courses. One has about 50 students, the other 40.  Both classes are made up primarily of juniors and seniors, a few sophomores and no freshmen.  One, the larger, meets at 9:30, the other at 12:30.  The material and topics are nearly identical, both are research methods courses.   The larger class meets in an auditorium style classroom, the smaller in a room with individual moveable desks. In both I use Powerpoint slides and primarily lecture.  In the larger class we only meet once a week for 90 minutes, the smaller class meets 2x a week for 90 minutes. But, the classes feel completely different to me.  The slightly smaller class is a lot more fun to teach, I feel more engaged, there are more questions.  Why is that?

Is it the time of day?
The size of the class?
The student composition?
The room?
The amount of time I spend in class?

I'm not sure, but it is interesting to feel the difference and makes me think about what I might change in my teaching for the future.