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In theory, they can be smaller because they project a smaller image circle.

In practice, the difference in weight and size is going to be negligible.

I can't find an apples-to-apples comparison in terms of EF-S to EF. What I have found is a comparison between EF and micro 4/3 lens. The Olympus 300/2.8 with a 4/3 image circle weighs 7 lbs, while the Canon 300/2.8 with a a full 35mm image circle weighs 5 lbs. I don't understand that one at all.

The Canon EF-S 18-200/3.5-5.6 is 21 oz; the Canon EF 28-200/3.5-5.6, 18 oz.

The Canon EF-S 18-135mm f/3.5-5.6 is 16 oz; the Canon EF 28-135 f/3.5-5.6 is 19 oz.

These aren't really fair comparisons because the range is different, but they are as close as I can find.

The biggest issue is the flange to sensor distance. That hasn't changed in the bodies with the smaller sensors, so the lenses still need to be of a retrofocus design to allow the mirror to clear. That's why comparable lenses are so much larger than with the non-retrofocal lenses used on the Leica M series, for example.