Good Discussion topic!
They should be based on swimming habits, bioload and size.... but are they? The recommendations we see in many different sources...where did they come from? Were they based on these 3 criteria?
Is there 5 different resources we'd be able to link here that we can compare to our own opinions here based on our experiences from keeping these fish. I think to really be able to have an true opinion you must have kept the fish at an adult size for at least a year. There's a difference in a 3 year old adult volitan and a say 7 year old adult lionfish.... people withold adults would provide more accurate information.
On a seahorse site I frequent, our recommendation came from years of keeping them... statistically you could see what the minimum size was were peopel still did well... the next size down and you'd begin to see the problem come in.