I mean the intensity of light per unit area at the film plane. Let's assume for discussion that we have two 210mm lenses, one intended for 4x5, one for 8x10. The one for 4x5 has a circle of illumination of roughly 6.5" (in reality I imagine it is larger than that, but this is just for sake of discussion); the one for 8x10 has a circle of illumination of 13", or twice the area. Using the standard definition of f16, the aperture opening for both lenses will be 13.1. Won't both lenses admit the same amount of light? Won't that quantity of light be spread over four times the area with the 8x10 lens?
As to the bellows compensation -- the light rays themselves don't lose energy over distance; they're being scattered over a larger area. Yes?