Whoever said 100 square miles is a small area for a city...it is not! That is actually a pretty large footprint for a mid sized city, average at best...that is the is basically the square mileage of Minneapolis and St. Paul combined(population 670,000 together). I am pretty sure Denver has open land within it's city limits...some of which was turned into suburban style development over the past decade...along with the land that was left to fill when the airport moved. All of that Denver growth was not in the core of the city, so don't try to make it sound like that
I am sure an impressive enough amount of it was though, with all the nice new developments.
Actually, a quick look at google maps shows tons of new suburban style development along I 70 towards the airport, that is within Denver's city limits. This is the kind of situation(and what that other poster was talking about I think) that benefits these large(sq mileage) cities growth. Along with densifying the core of the city, they can count on these suburban type areas to grow and increase the population(and tax base of course).
Growth is harder to accomplish for these cities that have been totally developed for decades...any growth has to come from redevelopment and building density while keeping all of the neighborhoods stable at the same time. Some small land area cities have done well, like Miami or Boston, or even DC.