It doesn't look very big, based on the size and number of windows, and the seat numbers (70-100) are much lower than the highest I've seen for the biggest jets (several hundred passengers). What am I missing? Is there some funky economic scaling effect that makes planes in a middle size range like this rare and unprofitable while bigger and smaller ones do fine? I never imagined that before but can see how it would be possible, but then I still don't know why it would be called "super"...

Smaller regional jets are very profitable for short to medium range flights, especially when serving smaller cities. It's a lot easier to fill 50-100 seats in a regional jet than it is to fill 130+ seats in a 737. Every type of plane has a minimum number of seats that must be filled before the flight is profitable. There are a lot of factors that determine the break even point: cost of the plane (lease or purchase), amount and cost of fuel consumed on the flight, the number of personnel on the ground and in the plane required to operate the flight, etc. Regional jets generally have lower labor costs due to having smaller flight crews (who are typically paid less than the crews on larger planes), smaller ground support requirements, lower fuel consumption, etc. If it wasn't for regional jets, my city (Colorado Springs) probably wouldn't have half the air service that we have today.

Using the spoke and hub arrangement so popular with the major airlines, regional jets provide service between the outlying smaller cities and the hubs. Flights between hubs or major cities typically have a lot more passengers so larger planes like the 767 or A320 make more sense. Really large planes like the A-380 are only profitable on the most heavily travelled routes (if then).