[Klaus]

I think there's a simple way to explain this.

 

Take a MFT camera and a FF camera. Let's assume that the whole sensor surface is sensitive to light. The sensitivity per square mm is identical.

 

Do not attach a lens. Same lighting conditions.

Open the shutter for -say- 1/100sec on both cameras.

 

Now "count" the photons that have been "gathered" by both sensors.

Which one gathered more photons and by which factor ?

 

Note: number of photons = speed.