Light gathering in M43 vs FF

In that example, one would either be overexposed or underexposed, correct?

Earlier, you also wrote, "What does f/2.8 1/200 ISO 100 on mFT do for you that f/5.6 1/200 ISO 400 on FF does not?"

As I understand it, exposure-wise, they should be the same. I've also read somewhere that in that type of example, the ISO100 of mFT should be the same as ISO400 of FF camera. But when I'm looking at an example from EM-5 RAW at ISO6400 and 5D Mark II RAW at ISO25600 in the dpr comparison tool, the EM-5 just seems to look better to me. I'm fairly new to photography, so can you explain this in a very simple manner?