Good side-by-side comparison shot.
I think there may be several reasons why the new arena may "feel" smaller:
1) - the half upper bowl in the Coliseum makes it seem larger than it is.
2) - in the Coliseum, the concourse is along the top of the lower bowl. In the events centre, the concourse is underneath the seating area, thus giving a less airy and expansive feeling to the building.
3) - I have a strong suspicion that the "pitch" (or incline) of the seating is steeper in the new events centre, thus making the space feel tighter than the Coliseum. Of course, a steep pitch also means that everyone sits closer to the ice surface........
You're correct though, the actual seats in the new events centre will be considerably larger than in the old coliseum.