Assuming two 20 megapixel sensors — one m43 and the other full frame, both get the same amount of total light.

The thing is they don’t get the same amount of total light. Think about two pieces of paper side by side. One is twice the size of the other (measured diagonally). Which one has more photons land on it?

Agreed that, if you’re looking for equal performance at the same depth of field, the FF gives up about a stop. But you have a choice as to whether give up some DoF to get the shot.

The real question is whether MFT is good enough for whatever it is you are doing. Sounds like it is for you, but that does not make them equal in low light.

Theoretically, the only ‘cheat’ I can think of to get around this is a lens adapter/concentrator that puts all the light from a larger-than-MFT optic onto that sensor. But once you do that, the weight advantage is gone, as the weight difference between the two formats is primarily that of the optics.

Then there’s low light autofocus. Do you know of any MFT cameras with PDAF units built into the sensor? I do not. PDAF (phase) far outperforms CDAF (contrast) in low light at the expense (in DSLRs) of fewer AF points. If there is a MFT sensor with integrated PDAF (like recent FF mirrorless), I’ll concede the point — and probably buy one as a secondary.

I do have to ask: have you actually used a FF camera in low light? When I switched from film to digital, I switched to APC. I can tell you that APC is nowhere as good in low light as FF, and it also captures less dynamic range, on APC I used to exposure stack as a matter of course, with FF it’s mostly unnecessary (unless I’m taking a shot from inside a cave to mid day New Mexico light — and I want to decide in post whether to crush blacks, blow out light — or tone map)