The funny thing is I didn't use compact mode before (although I did know it was there), and after upgrading to the new UI, the first thing I did was find it and enable it because of how enormous they made the tabs and buttons.
I run a large farm of VMs with telemetry turned on. I programmed them to make happy mouse movements whenever Mozilla embiggens their UI elements. My herd of shill-VMs has totally swamped out whatever telemetry-votes you compact-mode people are casting.