It depends too much on the variations in film technologies used by different manufacturers. For a start, you can't buy anything over 400 in sheets, and Fomapan 400 is very different from TMY2 in the grain department; much more of a difference than you observe between films of similar technology at different speeds (TMX/TMY/TMZ).
If you want to make a comparison without any real-world significance, using a notionally identical film technology with a noise floor that doesn't vary with speed, then the sensitivity you will achieve at a fixed SNR (signal to noise ratio, i.e. grain appearance) is proportional to the film area. Say you go from 6x7 to 4x5, that's a linear increase of 1.66x and area increase of 2.77x, which means that ISO275 on 4x5 should look like ISO100 in 6x7. Or that ISO400 in 8x10 should look like ISO100 in 4x5, etc. But there exists no such sequence of films with matching performance unless you look at a purely electronic sensor like a CCD.