Thanks for the kind words!
We initially planned to scatter some collectibles and secrets around the maps to help with that empty feeling, but unfortunately ran out of time there. I think if we were to continue development, we would also uniquely style each location and add in some alternative routes using the checkpoint system to make it a bit more complete and worthwhile to explore.
As far as balancing the map sizes: it's pretty tough, because every player approaches the mechanics differently! All of the levels except level 2 can be completed in about 30 seconds using some advanced techniques, and we had people's times clocked in anywhere from 30 seconds to 10+ minutes, so there's an argument to be made either way. I agree that it would be nice to have shorter introductory levels and longer challenge levels though!