Actually 5.25" were usually 360 KB, the 1.2MB came later, just like 3.5" floppies, they were originally 360 Kb, then 720KB, only later they became 1.44MB:
Though it all happened in a few years, when the 3.5 came out most if not all the 5.25 were already 1.2MB, and the first 3.5 were advertised for their robustness, not for their capacity (which was initially inferior).
For all the complaining I do about USB's naming standards—"Full Speed" is not in fact "full", it's slower than "High Speed"—I don't remember having the same gripes about Double Density vs. High Density :)
https://en.wikipedia.org/wiki/Floppy_disk#Sizes,_performance...
Though it all happened in a few years, when the 3.5 came out most if not all the 5.25 were already 1.2MB, and the first 3.5 were advertised for their robustness, not for their capacity (which was initially inferior).