Instead of trying to figure out what's happening by changing the sensor size, a fun hack is to instead think about it in terms of a teleconverter. The micro 4/3

Instead of trying to figure out what's happening by changing the sensor size, a fun hack is to instead think about it in terms of a teleconverter. The micro 4/3 format gives you roughly half the area of a full-frame sensor so let's think in terms of a x2 teleconverter. Let's assume you have a 200mm f/2 lens, and you put that x2 teleconverter on. The way you get the equivalent of 400mm out of that is that you're taking all the light that comes out of the back of the original lens, then spreading it out into a bigger circle. Same total amount of light, stretched thinner, so the sensor is gathering less light overall. That's why that 2x converter that turns your 200mm into 400mm also takes away 2 stops of light and reduces your effective aperture from f/2 to f/4. Now, the trick is that with the smaller sensor instead of the teleconverter, you have the exact same situation: You're taking that same amount of light projected by the optics, and cutting a smaller slice in the middle. You're just doing the slicing differently.