"But the flattop is a mid frame!"
Not all of them.
In the beginning,, the first OM .357's & .44's were Flattops, and on a medium frame. The Super Blackhawk came out with a larger frame,, and by 1963,, Ruger transitioned to the "protected ears" and larger frame. Gone were the original FT's.
Then, the New Model Flattops were introduced on the mid-sized frame,, in .357. BUT,,, due to the level of loads,, the .41 magnum, and the .44 Magnum Flattops were built on the large frame.
As our dear departed FlatGate would say; "Get the books!" (Meaning, study the history!)