As a Nikon and Panasonic shooter, I just saw the Sony A7 a few days ago and was quite surprised by the small size of it. I haven't done a lot of numbers on size and length of the lenses, but to tell the truth, the Sony lenses on display were not significantly different than those on my Nikon system (not much smaller) and quite a bit larger than my m4/3 lenses. I agree that the size /numbers given by Paul would suggest that there isn't much difference, but from what I saw, there certainly is. I just looked quickly at B&H and Panasonic zooms to find two comparable lenses. The closest I could get was the Panasonic 14-140 (equiv. 28-280) f3.5-5.6 and the Sony 24-240 f3.5-6.3. The Panny is 2.64 inches (67mm) in diameter and 2.95 inches (75mm) long. It weights 265 grams. The Sony is 3.17 inches (80.5mm) in diameter and and 4.67 inches (118.5mm) long. It weighs 780 grams.
So the Sony is wider, not quite twice as long, and weighs almost three times as much. None of this, of course, says anything about build or use of plastics vs. metals, although I have not been unhappy with my Panasonic lenses. Also, I'm not sure that B&H shows all the Sony lens offerings, but they appear to have no zooms faster than f4, and the lens selection appears to be much more limited. I could be mistaken about this, and might not have seen all the lens offerings.
I also suspect that the difference between f2.8 and f4 has become a lot less important than it was in the film days, because high-ISO quality is so much better. For most people, that one-stop difference in lens speed may not be as important as the reduction in weight.