Biggest problem with large spindle is that noise and heat dissipation (both mostly due to air friction) are roughly proportional to the square of linear speed of the outer part of the platter, and bigger the platter, more linear speed you'll get at the same angular speed. On the other hand, you want to crank up angular speed to get any decent random IO operation/second value.
Last 5.25" HDD I've seen was Quantum Bigfoot CY [1], (which happened to be half-height FWIW), and this thing was pretty noisy even at 3,600 RPM. Dividing this by 60 (seconds in one minute) gives you theoretical maximum of only 60 random IOPS. Not exactly stellar, and as far as I remember, that was substantially lower in practice.