Did it ever occur to you that the surrounding population has a great impact on stadium size? Boise for instance probably draws a far higher percentage of the available population than most of the places you mentioned. And by available population I mean within reasonable driving distance. The size of the student body is another determining factor. The entire state of Idaho is about 1.5 mil which is much smaller than the greater Detroit area by itself. Boise will increase capacity by another 50% within the next few years and will most likely be sold out even then.
Yes it did occur. However many other school and pro teams have large stadiums while have low population in the surrounding area.