Weight: system more important than sensor size?

I think the main thing is, that lenses need to be better for FF digital (especially for let's say 36MP) and lens size is one of the most important restrictions for lens designers; i.e., to make a lens better, it will have to become bigger also... If one is allowed to make a bigger lens, it is much easier to make it very good also.

(Actually 4/3 is a very good example of that, and it is my opinion this has been one of the key reasons Olympus has chosen a smaller sensor size: they realized that lenses would need to become better, and thus would become bigger. A smaller sensor would need lenses more or less "normal" size or a bit smaller, but next-gen FF lenses would need to be enormous. The 50mm Zuiko has ELEVEN lens-elements. Making a 100mm f/2 that is so well corrected.... well you get the point, even ±100mm f/2.8 versions that are available are 3x the size.)

I have been a professional photographer for >20 years and every now and then I open an old slide scan on my computer made with let's say a Nikkor Ais 28mm f/2.8, pro-grade in any way when it was new. (still have that lens btw) and to be frank I am sometimes shocked at the image "quality" that we accepted as "very good" back then. Using a modern DSLR (especially a FF one) with legacy glass is cheap (well, not even that actually) and it is simply stupid. You really NEED for example the latest Canon 24-70 which is twice the price of the old one, because it is corrected better.

To amaze yourself you could have a look at the Carl Zeiss Digiprime cinema-lenses. They cover about 4/3-size, and have quite "normal" specs like 25mm f/1.4. Google for a few images to see how big a lens-design becomes, if image-quality has to be EXTREMELY good.

On the plus side it seems manufacturers are making more and more slower (f/4-ish) lenses, probably because high iso has become so good...

Lourens