Salt Lake City's boundaries are huge, but a good chunk of that lies in an area that's uninhabitable due to poor ground. This is mostly everything west of 215.
Let's remember, Salt Lake stretches all the way out to 8800 West over in Northpointe by the Great Salt Lake.
To put that in context, this area is just as west as Magna, though obviously to the north.
Which is why Salt Lake's density numbers are always deceiving, because they incorporate not only that western swatch, but also a huge area of mountain north-east of the city (diagonal from the Avenues).
You can't build on this area. So Salt Lake's buildable land is essentially already developed (everything east of 215 to the mountains).
Which means you can probably fit what has been developed into the incorporated part of the city and
still have room.
So Salt Lake is 110.4 square miles and Boise is only 64 square miles.
However, that contains most of Boise's urban area -- land that has been used for city development. Not all, mind you, but a great deal more than Salt Lake.
If Salt Lake City's proper was solely the area of the city built up and not the huge area of land west of the city that will never be developed, it would be about 45 square miles of land -- maybe even less.
That's a pretty stark difference and explains why Salt Lake's population has never pushed beyond 200,000.
To get beyond that point, it's going to take larger and more denser developments. Because, outside of a few locations on the western side of the city, there just isn't anymore area to grow -- unless you grow up.
To illustrate my point more, here is the map of Salt Lake's boundaries.
As you can see, a lot of land there that has yet to be developed or has only been developed by light industrial.
I don't know many other cities with less than half their square miles developed.
I'm sure they're out there, but not many -- or at least many as big as Salt Lake (both in population and size).