Why do you say "With larger formats obviously the factor comes down somewhat - 6-8x maybe for medium format, and 4x or 5x at the most for 5x4".

Assuming the film is the same why should a Hasselblad with a Planar, for example, not allow as much a magnification factor as a 35mm? Similarly for 4x5. It comes as a bit of a shock to learn that I can enlarge to 25" from a 35mm (x17) but only 20-25" from a 4x5 (x4 or x5) or 13-18" from medium format (x6 or x8).