So I wasn't imagining that the 04 looks narrower, since it actually is. Does this affect cabin room at all?
Not much, some larger in the 2004, some smaller. Here are the values from the Pontiac website - correctly typed this time
Dimension.............2004.....2003
Front headroom......38.8.....38.3
Rear headroom.......36.2.....36.5
Front legroom.........42.2.....42.4
Rear legroom..........36.2.....35.9
Front shoulderroom.58.0.....58.5
Rear shoulderroom..54.8.....57.2
Front hiproom.........54.5.....55.7
Rear hiproom..........54.3.....54.3