This place is crowded. Is it just much more popular (and even moreso given the Northeast corridor advantage Boston would have had) or is the arena just smaller than it was in 2004. Nosebleed seats didn't really fill up until later in the week then. It's early in the night on Monday.
To the Google! What was then the Fleet Center but appears to have had a named change has a maximum 19,500 maximum capacity, though probably seats behind the stage were blocked off.
The Pepsi Center feels a bit smaller than the Boston arena, though given that the former has a basketball (not maximum) capacity of over 19,000 my impression is probably wrong.
So, yes, things are a bit crowded. The Thursday move to Invesco Field is wise, because given the various ways people engage in credentials shenanigans this place would have been bursting.