[rayjay]

Need isn't all based on volume of tank but whatever it takes to keep the bacteria at bay mostly. Tank size choice should be made based on expected adult full size, not their current size as juveniles, although when they are small, you can obviously still use smaller volumes, increasing as they grow.
IMO, 5" adult seahorses still need 30g/pair.
3.4" adult seahorses need probably 20g/pair.
HOWEVER, if a person knows that they can maintain the rigid protocol to keep the water such that bacteria DON'T become problematic, then that's their choice.
The UNFORTUNATE thing is that some hobbyists will start out being sure that they can maintain a certain protocol but as time goes on, some will end up skipping a cleaning/water change here or there and over time, the accumulative result of the missed tasks allow for the nasty bacteria to become plague proportions.
It would be GREAT if we were able to have test kit(s) that can tell us when the water is approaching that point, but there IS NOTHING available to the hobbyist to be able to indicate that so everything has to be done pro-actively rather than re-actively.
I have one tank that is severely overstocked with seahorses, BUT, I clean the mechanical filters every 3 days and do at least 90% water change on the bare bottom tank ONCE A WEEK, without fail.
As for sizes of seahorses, IME, the largest reidi I've had was probably only just a touch over 6" and the largest erectus probably a touch over 7" and that took several years for them to get to that size.
Currently, my barbs at 2 yrs of age are averaging about 4.5" and the 2 yr old abs are only about 7-8" so I expect it might take another year (or more) for them to max out their size.