Current HDD tracks are around 50-60nm wide, and their limitation is not magnetic grain density (those are around 10-12nm for non-HAMR/MAMR substrates and even smaller for HAMR/MAMR substrates.) The limitation is the long (relatively speaking) actuator arm trying to precisely stay on a very narrow data track. There's actuator flexure, disk flutter, and aerodynamic noise, all of which increase with increasing platter radius. This increases your minimum track width, and minimum disk thickness/minimum disk spacing and end up defeating the purpose of bigger disks in the first place.
Also it's important to keep in mind that in order to manufacture a drive, the entire drive is written to and read back multiple times, which takes an increasingly long time, creating a huge push for some steps to be combined.
That all being said, I'm not entirely sure where the sweet spot in terms of data density per unit volume. It might be slightly more efficient with 5.25 drives. But I can assure you that the record sized or washing machine sized HDDs of olden days are not practical anymore ...though they would be dope if they did exist!