It’s called ECONOMICS.
Any decent eduction will offer this, to varying degrees.
In a market system, when demand dwindles, the supply will be restricted.
If you want specific proof, check out the McDLT.
They offered it, people didn’t buy it, so they stopped offering it.
Whoda’ thunk it?
Your argument seeks to change the subject. The subject is SIZE of portions, not specific items. Further, you asserted that it was a proven fact. So far, you have failed to provide the specific proof.