The inverse square law works, but as you get bigger, some other factors kick in. One is reciprocity. With exposure times greater than a minute, it can be a considerable factor. With really long exposures, reciprocity can affect contrast as well as exposure time. Another is flare, which can lower contrast and generally make things muddy. As you get further away, there are just more things for the light to bounce off of, and there is more chance of stray light and safelight fog affecting the image. Condenser enlargers may work better than diffusion types for big prints.