Exactly, there is underlying physics involved. As sensor size changes you are trading off and balancing factors such as pixel pitch \ gain \ dof at a given ap \ fov at a given focal length \ diffraction (coc) etc. A lot also depends on where money has been put in each system. Digital large format is virtually solely scanning backs used for landscapes, cameras are huge, lenses are actually pretty light considering the image circle involved. Medium format means you have lighter cameras, moderate lenses, you start getting af and even IS now. They also tend to be ccd rather than cmos so you find yourself overshooting, dealing with 1fps max normaly but true 16bit. 35mm tends to be cmos so you get liveview, higher isos faster frame rates (although sensor size is part of that) etc. Inpart due to the physics of the medium but also because thats what manufacturers built. You could have an 8x10 digital slr with IS AF lenses but it would be huge and insanely expensive and smaller fov lenses with wide aps would be monumentally HOOOge.
Camera sensors win on convienience, theyre starting to get IS and even optical zoom. Diffraction will play a part with clarity and forget thin dof, but I won't complain about them improving :-) Horses for courses, but don't believe the whole 'takes dslr quality shots' marketing balls. I'd happily switch away from insanely expensive and heavy gear if they did.