There have been multiple threads about supposed and real "crack downs" on carry bags. There was a fairly major effort in 2014, though it was short lived and sporadic at best.

http://www.flyertalk.com/forum/unite...ar-2014-a.html

http://www.flyertalk.com/forum/unite...t-works-2.html

Newest rumors:
http://www.flyertalk.com/forum/unite...nfirmed-3.html

More on topic regarding the Group sizes, it appears to me to be high variable based on days, times, calendar, etc. etc.

I was through DEN this weekend and saw a group 2 line that was HUGE. Largest boarding line I have ever seen, snaking well out of the gate area. Unusual enough that I made special note of it.

I don't think random observations are going to be of much use.