Thank you Mebbid and Justin for your replies. It really helps get others thoughts when you just don't know.
I was looking at the various Lionfish breeds that are common to this hobby and was wondering why the tank size was on average 75 gallons for the Volitan specie (120 on LiveAquaria) since I'm used to these fish being relatively slow movers, often times just hovering. Then I started looking at the length of the tanks and not until you get to the 125G size do the tanks get any longer, so that led
me to wonder if the fish just needed the water system volume, and more generally to my original question, how do you know why.
After posting I also found a video of a Volitan in a 4' tank (probably 55G) and then I realized how big a 15" Lion really is, poor guy. Also, to note the 120G recommendation from Live Aquaria get the fish in to a 6' long tank, which may explain how that site decides its recommended sizes.
Anyway, I'm sort of rambling now so ill stop.