which is 80% a sales thingie and 20% an actual useful feature.
and without going into much technical discussion (there is a link to an excellent lecture on sensorsizes vs pixeldensity for those interested here:
Demystifying Digital Camera Specifications) the summary is, more pixeldensity (MP) on a small sensor size is not all nice and well.
So even if the files become bigger, you are talking about two different horses when it comes to the clarity, dynamic range and detail that you will capture between a medium format system and a small format system.
And for those of you who are old enough to remember the ancient times when we used film, the exact same discussion were rampant back then aswell, discussing if extremely low iso film on a small format could compare to a medium format
The answer was no back then aswell.