Could the large size of the Dearborn station be due to the fact that it may be a main amtrak station for the entire Detroit area? I mean this doesn't scream "main terminal" in Detroit proper. Not to mention this Dearborn station is situated on the west side of Detroit, towards Chicago where I assume most trips will be to/from in the entire metro.
http://www.detroit1701.org/Graphics/Amtrak.jpg
I imagine if St. Louis, MO were flipped into Illinois, the middle ring suburban station in Kirkwood would be
a lot bigger on the line to Chicago instead of being a station to KC.