if you do the maths, depth of field depends on both the aperture and the sensor size. in other words, f1 on a smaller detector doesn't give the same depth of field as on a larger detector (for an object at the same distance).

i wrote about here, but i am not sure it's very clear http://www.acooke.org/cute/DepthofFie1.html

[edit: and so, for medium format, depth of field is more pronounced...]