A 70 mm lens for 645 and one for 35mm cover different negative areas evenly at the focal plane due to design, not actual theory involved in optics. It is a matter of economics.
This also involves the plane of object focus of a lens which is nominally curved and must be designed for the given film size. Again, this is a matter of economics rather than theory. So, a 35mm lens is designed with a smaller area of coverage and a smaler area of focus compared to a similar lens for 645.
A lens for a large camera can more easily be adapted to a smaller frame camera than the other way around.
At least, this is a practical rule of thumb I was given when I was learning these things.
So, it is a matter of practice and econimics vs what is theoretically and really possible given an unlimited budget.
It can be demonstrated by focusing a scene from two 70 mm lenses on a sheet of paper outside of a camera and observing the circle covered by both lenses on the paper. This is how it was demonstrated to me at one time.
I'm not an optics person, so I'm only reporting on observation and what I was taught pursuant to other elements of system design.