I kaypo here, I think the Full frame term in 35mm got it's name because when digital SLR 1st introduce, there were no lenses designed for smaller sensor, the engineer tried to imitate the 35mm film camera design and structure. They did a prototype and proof on concept based with a film camera, replace the film compartment with digital sensor. Then the 1st digital SLR created. All the lenses based on 35mm film SLR able to project direct to the sensor without crop.
After years of research and advancement APS-C DSLR were out in the market, it is smaller and cheaper to produce. When we mount the lenses design for 35mm film on this generation of DSLR, the image projected on the based on the old design will get some crop. So everyone call it crop sensor.
Then our "O" brand said they wanna redesign and create a new standard for digital SLR, the japanese just go back on drawing board, redesign the whole range of sensor, lenses and body. There you go, now we got a whole set on lenses that can able to project full image on a smaller sensor. So in theory 4/3 it is a digital full frame.