This was a problem last year in Madden 11 as well. I don't know what happened but they seemed to do something within the coding to make the stadium half empty.
Now this would be good if they brought back Owner Mode and the stadium was half empty because of the team losing over a period of seasons. Half Empty for playoff games and constant winning is a shock, those stadiums should be packed like you said. This is something that needs to be fixed within the next patch but certainly something for Owner Mode and to represent how fans feel when your losing.