It's true that on the long end 35mm full-frame lenses are significantly larger and heavier than micro-four thirds lenses. However, most people don't shoot extremely long lenses most of the time (and when they do they can afford to use a lighter f4 lens because larger sensors have better noise peformance). Compare Tamron 28-75 f2.8 for Sony E mount (550 g) with Olympus 12-40 f2.8 (382 g). That's only 168 g (44%) more for a lens that lets in >3.5x more light. If you wanted to take portraits with very shallow depth of field with Olympus (possible with the Tamron lens), you would have to buy a dedicated prime (and one could always find a brighter prime, often for a used one for much less money in the 35mm full-frame world).
Even on the long end, advancements from Canon/Nikon/Sony (such as DO and PF lenses as well as very high-resolution sensors with excellent noise performance) meant that Olympus' advantage was disappearing. For example, all 300mm lenses with the same aperture share the same design and similar size. And yet 300mm f4 from Olympus weighs 1,474 g while 300mm f4 PF from Nikon weighs only 755 g -- twice less. Yes yes it's true that the Olympus comes with a 2x magnification factor. However one can make that up by using a camera with a very high-resolution sensor (such as the one in Z-7) or simply by using a teleconverter and taking advantage of higher noise performance of the full-frame sensor.
Ultimately the reason micro-four-thirds lenses were lighter was not because the system was better -- they were lighter because they were dimmer. The fact that this was not mentioned in most marketing materials made Olympus and Panasonic look quite bad -- and many interested prosumers (like myself) had felt the company was lying to them.
Even on the long end, advancements from Canon/Nikon/Sony (such as DO and PF lenses as well as very high-resolution sensors with excellent noise performance) meant that Olympus' advantage was disappearing. For example, all 300mm lenses with the same aperture share the same design and similar size. And yet 300mm f4 from Olympus weighs 1,474 g while 300mm f4 PF from Nikon weighs only 755 g -- twice less. Yes yes it's true that the Olympus comes with a 2x magnification factor. However one can make that up by using a camera with a very high-resolution sensor (such as the one in Z-7) or simply by using a teleconverter and taking advantage of higher noise performance of the full-frame sensor.
Ultimately the reason micro-four-thirds lenses were lighter was not because the system was better -- they were lighter because they were dimmer. The fact that this was not mentioned in most marketing materials made Olympus and Panasonic look quite bad -- and many interested prosumers (like myself) had felt the company was lying to them.