The changes in Stilwater between Saints Row and Saints Row 2 are as if Saints Row 2 played like 30 years later other than 5 years.
Saints Row 2 added the University District, Mourning Woods, Trailer Park, the Suburbs Expansion District, the nuke plant, the prison island, the smaller easteregg islands, the airport is about twice as big as before, the Hotels and Marina District, the underground mall, the Stilwater Caverns and the Saints Row district is changed drastically. Also Mount Caflin was inaccessible.
They also fleshed out a lot of districts by adding more detailed decoration. Oh and there's also a new bridge that hasn't been there in Saints Row, the one east of the Ultor Dome.
There's probably a lot of other changed stuff that I didn't think of now, but as you see the list is long.
I somehow can't imagine a Steelport that undergoes changes like that with new islands and districts added.