There are two ways to aproach photomacrography.
One is the 'frame-drive' approach, thinking of a frame as something to fill as its first goal.
Then you do indeed need higher magnification (= less DOF, but not necessarily more diffraction - depends on how much you stop down, of course) when switching to larger formats.
The other is the 'magnification-driven' approach, having a certain scale, and with it a certain level of detail, as its goal.
In this approach, larger formats mean more 'real estate' to fit the subject in.
Since DOF depends on magnification, not on format size, you do get the same DOF (assuming same f-stop). Diffraction then too does not change.
Exactly. If you want to shoot large format, you need a large subject. Though, I suspect most people interested in 'macro' photography want to take a picture of something small