The size of the board has little to no bearing on the quality of the VRMs. VRMs are VRMs. There's nothing that prohibits an mITX board from having the same VRMs as an mATX board or even a Full ATX board: The reserved CPU area still applies to all board sizes. The extra width is mostly beyond the memory slots or below the first PCIe 16X slot. That has no bearing on what's used for the CPU itself.
I would also wager the vast majority of people who have a PC only use one SATA or NVMe socket/slot and only have a GPU as an add-in. This makes, for the most part, an mATX board mostly as used as an mITX board in terms of features.
My first generation AM4 mITX board that I built with a Ryzen 7 1700X can handle the 3000 series Ryzen 9 CPUs just fine.
I would also wager the vast majority of people who have a PC only use one SATA or NVMe socket/slot and only have a GPU as an add-in. This makes, for the most part, an mATX board mostly as used as an mITX board in terms of features.
My first generation AM4 mITX board that I built with a Ryzen 7 1700X can handle the 3000 series Ryzen 9 CPUs just fine.
https://www.asrock.com/mb/AMD/Fatal1ty%20AB350%20Gaming-ITXa...
Even the Ryzen 9 3950X.
I recently did a rebuild and got another mITX AM4 board.
https://www.gigabyte.com/Motherboard/B550I-AORUS-PRO-AX-rev-...
It handles all Ryzen 3000, 4000 and 5000 CPUs. Including the 16 core Ryzen 9 5950X.
Size =/= Quality of Board.