Not exactly how microwaves work.

Microwaves excite molecules, especially things that are the same size as the wavelength (1mm-1m depending on the frequency), i would guess that the plate being bigger than the food and full of impurities, it's molecules absorb more waves/energy.

Or it's just that that person's microwave has a lot of standing waves at the very bottom of the pedestal.

A fun thing to look into: Electrobooms microwave standing wave video and the one where they create plasma in grapes (grapes that are the same diameter as the wavelength)