I think you've got it right here. The 20,000-seat behemoths were a 1990s thing... the pendulum is swinging back in the other direction. In addition to raising the average ticket price, another factor is the increasing number of suites.
Here are the arenas built in the past 15 years, with capacities listed. The decline in capacity is apparent:
Bell Centre - 1996 - 21,273
St. Pete Times Forum - 1996 - 19,758
Wells Fargo Center - 1996 - 19,537
Scotiabank Place - 1996 - 19,153
HSBC Arena - 1996 - 18,690
Bridgestone Arena - 1996 - 17,113
Verizon Center - 1997 - 18,277
BankAtlantic Center - 1998 - 19,250
Air Canada Centre - 1999 - 18,800
RBC Center - 1999 - 18,680
Philips Arena - 1999 - 18,545
Staples Center - 1999 - 18,118
Pepsi Center - 1999 - 18,007
Nationwide Arena - 2000 - 18,144
Xcel Energy Center - 2000 - 18,064
American Airlines Center - 2001 - 18,532
Jobing.com Arena - 2003 - 17,125
Prudential Center - 2007 - 17,625
Consol Energy Center - 2010 - 18,087
Just to round out the list, here are the hockey capacities for new NBA arenas not included above:
Conseco Fieldhouse - 1999 - 18,500
New Orleans Arena - 1999 - 18,000
Oklahoma City Arena - 2002 - 18,036
AT&T Center - 2002 - 13,800
Toyota Center - 2003 - 17,800
FedEx Forum - 2004 - 11,411
Time Warner Cable Arena - 2005 - 14,100
Amway Center - 2010 - 17,200
Personally, I'm thankful for this trend. I usually find the smaller arenas to be much more fun than the mammoth ones.