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.

Maybe the kind of people who use compact mode turn off usage statistics, and those who use pocket (non-accidentally) don't turn them off.

I wonder how many users who disable telemetry are also using compact mode and how many people are actually disabling telemetry.

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.