I realized something this morning.

With 3.5" floppy disks, unformatted HD disks are 2 MB, & DD disks are 1 MB, so HD disks have twice the capacity of DD disks.

But with 5.25" floppy disks, unformatted DD disks (assuming double-sided) are 500 KB, & HD disks are 1,500 KB, so HD disks have 3x the capacity, not twice.

Later TIL there was a seldom-used "quad density" between DD & HD in 5.25" standards.
https://en.wikipedia.org/wiki/History_of_the_floppy_disk