Largest City in South Dakota?!

Last night I drove on through to Mitchell, South Dakota.  On the way I passed through Sioux Falls, which had a sign saying "South Dakota's Largest City"... two seconds later I blinked and I was outside of the city.  If that's the largest city, I'm beginning to wonder what the rest of the state looks like!  I just got some tips from the locals on what to do today - let's just say it is going to be a busy day and almost all of it will be spent in South Dakota exploring.