medium format revisited

[efff]

Hi everyone,
I have a question I never found an answer to (on the net, at least). Maybe smart people who usually occupy forums will help me out with this: most of us know that medium format film offers finer gradation,..., than smaller-format film. Everyone knows that this happens when enlargements are produced (this is straightforward to understand). What I don't understand, and maybe it's not the case, I don't know, that's my question, is this: as long as the chemical emulsion is the same regardless of the format (maybe it does vary with the format(?)), the development process is the same regardless of the format, and everything else when shooting the scene is the same (the scene, the distance, the aperture,...), why there is a difference in the gradation, when under these circuimstances the smaller format should only capture a cutout of the scene the larger format will capture. That cutout should be the same in terms of quality. So why is there any difference provided there is any difference? I am not talking about any enlargements now. If there is a difference in quality, is it he camera, larger lenses, different chemical substance to coat medium format film as opposed to 35mm format,...I dont know. Maybe this is such an advanced chemistry that only those who invented it know the answer. The thing is I am the kind of guy who needs to understand how things work or else I dont pay attention to them. IF EVERYTHING IS THE SAME, THEN THE ONLY DIFFERENCE SHOULD BE THE FRAME SIZE OF THE SCENE, this is what the logic says. If it is not the same, then where does the difference come from? This a comparison of film vs film, not a film vs its discrete approximation called digital, of course.

P.S. I shoot digital, but I am considering film as well.

THANKS !!!