Practical differences between two adjacent format are never as clear as black & white. And when shooting in good light, the differences are even smaller. Also, depends on how fast is the lens, when smaller sensors are used in combination with faster lenses, the difference of image quality gets smaller. But it's a bit like comparing a small car and a big car for driving downtown, since the speed is limited below the max speed of each car, there is not advantage of using the big car and the smaller car is easier to park. Compare the two cars when driving 500 miles of highway, the bigger car will made a lot of difference for the whole journey: you'll arrive earlier at destination with the big car, it'll be safer, and you'll be less tired when arriving. Anyway, we should always consider the personal interest of the person doing the video, he is teaching photography, therefore his interest is to have as many students as possible including people with smaller cameras, spending the money on camera equipment or spending the money for attending workshops are competing activities. Anyway, full frame is the sweet spot for stills, wide variety of glass, lots of f1.4 lenses available making it possible to shoot at low ISO more often than with other systems. Full frame isn't the cheapest, but deliver the best image quality for the money.
---------- Post added 09-11-18 at 10:23 ----------
Originally posted by aurele
Unless you do "ultra-prints" (huge prints @600dpi), you'll never see big difference. Well, some people might (@Digitalis maybe ? ) with trained eyes. But the good news is most people who buy prints won't ever notice.
The prints in the video are too small. The video is aimed at hobbyists by someone offering workshops for hobbyists, hobbyists seldom print and they seldom print large. The OP is also a hobbyist. If you have a m4/3 camera it feels good to watch the video. If you have a medium format system, you probably never have time to watch the video, because you are too busy with commercial jobs, and setting up the workshop for the amateurs of m43.