Electron beams scan insanely fast, that isn’t the limiting factor. Getting that much bandwidth across a VGA cable is tough. If you wanted super high refresh rates on old CRTs you’d have to drop the resolution. Same concept.

Changing the resolution on a CRT normally doesn’t make the picture smaller. There is no native resolution, phosphors are not pixels. My Viewsonic would display 640x480 or 1600x1200 on the whole 21” regardless. You can also watch the video, it’s not using a smaller area.

I believe the limitation is bandwidth, not the electron beam.