Light gathering in M43 vs FF

Suppose you make two pizzas, one small and one extra large.

One pizza is bigger than the other right?

Now measure the thickness... both are the same size.

So it depends how you look at it.

Aperture is calibrated differently on different lenses (the physical size of the hole varies) with the express purpose of delivering the same 'thickness' of light for the image circle it produces compared to any other lens at the same aperture. Thus a large sensor at a given aperture gets a larger total volume of light on an 'area' basis but it gets the same amount of light on a 'point' basis as a smaller sensor. Thus exposure behavior remains consistent for both sensors but a larger sensor provides a better quality image because it gets a larger volume of light.