I also think these corridors are all very small, which surprises me a bit, since you seem to have so much space left in the portion below.
In any case (if you want to keep it that way), I would suggest to move your whole level a bit more down at least and make the walls on the upper half higher or something because it seems quite easy to skip pretty much half of the level.
I really like your Background though. Looks pretty great and I think it works well in SMW.