I really don't see why we need more lighter EF-S lenses when there already are a bunch of very good EF lenses that cover a wide range of focal lengths and work on all cameras.
I second that. There are already some (really!) fantastic EF_Primes out there like the 40mm 2.8. You can't make it much smaller even if you want to.
The thought of making "smaller" Lenses for APS-C only (on Canonbodies) is a wrong one, because the Bayonett (and so the diameter) plus the Flange focal distance are the same. If you design a new Camera with a smaller bayonett like an EOS-M you could get advantages in size, but as the EF-S is just a trimmed EF-mount... you get all the downsides but no real gain.
The only chance is to get shorter lenses if they reach into the mirrorbox.