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.
Well, you probably could, but then you would have to choose between being able to manually focus the lens and being able to grip it to attach it and remove it.
But seriously, yeah, that's one nice piece of engineering, IMO. Making medium to wide lenses smaller than that almost certainly isn't very useful. Making long lenses smaller, however, is useful if you can pull it off without losing too much image quality.
Any L lens will perform just as good is not better on a crop as you won't get as much of the soft corners people always complain about.
That is true only if the crop body's resolution is areally proportionate to that of the full-frame sensor—that is, if the crop body is the equivalent of cropping a full-frame shot down to APS-C size. In practice, however, that's almost never the case, because nobody wants to buy an 8MP crop body these days.
A given lens can only resolve features up to its angular (spatial) resolution. So suppose you have a lens whose resolution is barely good enough for a full-frame sensor. Assuming that the crop sensor has the same number of pixels as the full-frame sensor, the pixels are smaller which means that the circle of confusion covers more pixels on the crop sensor. As a result, if you use foot zooming to get an identical shot on a crop body and a full-frame body (ignoring parallax differences for the moment), the full-frame shot would be, on the whole, sharper than the same shot taken on the crop body.
The fact that you're using the sharper, center part of the lens mitigates that difference somewhat, of course. The result, as I understand it, is that the corners tend to be sharper, but the center is much less sharp. Of course, if the center of the lens is way sharper than it needs to be for a full-frame body, then you'll get a sharper image overall. In practice, this is usually not the case, however, because when you design a lens, you can only get more sharpness by giving up something else (e.g. by making the objective lens bigger and bulkier, which also makes the lens and filters more expensive).
Perhaps...but the point seems rather moot for the original discussion concerning EF-S primes, since there's only one. Also, I wasn't questioning that an EF-S lens could be smaller and lighter than the same focal length/aperture in an EF lens, but rather your figure of 'two-thirds' the size.
My point was that you could
make them that much smaller, not that any manufacturer necessarily would.
It's probably better to compromise between making them smaller and increasing the resolution.