Light gathering in M43 vs FF

It is not measuring the gain. What it measures is the saturation exposure or 'light capacity' at that ISO setting. What it really is is the minimum ISO you could use at that ISO setting, with only the highlight headroom built into the ISO Exposure Index definitions. Most manufacturers (notably excluding Panasonic) allow something aver and above this in their metering calibration, which is why DxO 'ISO' measurements look lower, in general than the nominal ISO, the E-M5 giving pretty much a stop on top of the ISO headroom. If the meter is properly calibrated and the processing a correct match, and you set 100 ISO, 100 ISO is what you'll get in the photo, regardless of the sensor's saturation exposure at that setting.

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Bob