Light gathering in M43 vs FF

It counts for exposure, but that's because ISO is not a constant but a variable between cameras. On m43 cameras (or any digital camera for that matter) the ISO settings are made so the exposures work out correctly allowing for the sensor performance. The way to look at it is you get 1/4 the light on a m43 sensor over FF so they just turn up the gain at any ISO setting so the exposure is correct for that ISO.

The full-frame sensor in the example will have the same field of view but capture 4x the light. The amplification in the camera will adjust it so ISO 200 needs the same exposure but you will have a lot more light, so all things being equal a lot less noise in dark shots.

If you took a m43 sized crop from the FF sensor it would look similar to a m43 pic taken with the same focal length lens at the same exposure.

However that's not what happens, what happens is you take your photo and show it at a certain size, say on a 24" monitor. The FF shot captures 4x the light in that photo. Basically you can think of it as taking 4x the light and crushing it down to m43 size squeezing out the noise.