It's only on the wide angle lenses that you see a little bit of length difference; as the FL increases to general purpose and telephoto lengths, there is no size benefit at all, and 100g is a variance that you see from Sony to Canon EF lenses one way or the other, keeping in mind that the internals are not the same; one uses STM and the other has mechanical focus, the manufacture materials, especially on the f/4's can be quite different.They will likely be smaller and lighter. Sony's f/4 16-35 is smaller (1" shorter) and lighter (by 100g), and it performs better than the Canon version. That is significant.
Specifically on the wide angle trinity zooms, the size/weight of a 16-35 (whether f/4 or f/2.8) was never an issue. for me. I mean, I could hold one all day on a full size DSLR body and never tire; even so, I'm not really sure why I would such a wide lens. Starting at 24-70 and especially at 70-200, where I am likely to get a lot more hours holding it at eye level, the f/2.8 get heavier and larger than f/4 lenses, to the point where I often prefer the f/4 for weight reasons.
If there were a way to make a 70-200/2.8 IS , without any sacrifices, in the size and weight of a 70-200/4 IS, I would preorder one in a heartbeat even if it cost four times today's price for 70-200/2.8, but I literally think this is impossible without a revolution in optics.