Whoever came up with the term 'Full Frame' deserves to be strapped in a chair for days and forced to listen to Ed Sheeran non stop. (Personally I think that is probably the cruellest torture anyone could devise!!). Because what does it mean? And where do larger sensors fit in? Are they Full Frame Plus? APS-S sensors are aptly named because the are the same size as the film system of the same name. So why isn't 'Full Frame' called by it's correct term which is 35mm, because the sensors are the same size as a frame of 35mm film.
However, for whatever reason, somehow this 35mm film sized sensor seems to have become the sensor size that is regarded as the 'standard.' But is there any evidence for this. In an interview on Dpreview the VP in charge of sales and marketing at Olympus US claims that 'In the US, full frame is 18% of the market.' Now presumably that is the interchangeable lens market, including mirrorless and DSLR cameras. However, when you put that into context with all time global sales of any type of camera, then that % comes right down to a fraction since most cameras sold are on smartphones.
There is also the assumption that somehow 35mm / "full frame' is a superior format in terms of quality. Again it isn't the best possible, since larger sensors obviously capture more detail, but somehow it has become assumed that 35mm / "full frame' is something that serious photographers and professionals aspire to. Back in the days of film, that certainly wasn't the case. Yes 35mm was easier, smaller, lighter and cheaper, but if you were seriously inclined to produce high quality images then Medium Format film was required. 6x4.5 cms, 6x6 cms, 6x7 cms and 5x4 inches were common formats.
OK, most of us no longer work with film sizes, so let's look at it another way. Pixel size. Manufacturers are cramming more and more pixels into that 35mm film sized rectangular. Sony are up to 61MP. That should mean smaller pixels and therefore poorer high(er) ISO performance. But that is not necessarily always the case. Because digital camera sensors are getting better, to a large extent because of the need to make those small smartphone sensors better. Fuji and Panasonic / Olympus have certainly made enormous strides in terms of the quality of their smaller sensors and even m4/3 now has very decent performance up to ISO 6400. The image below with a 100% blowup, was taken hand-held on my Olympus E-M1X in the high res mode, so it's 50MP. It was also so dark as to require 1/15th. sec. at f/1.7 on my Lumix 10-25 f/1.7 lens at ISO 6400. So where's the noise that's suppose make m4/3 an inferior format? Lets see a full-frame camera do this significantly better.
