I totally agree volume is certainly not the defining factor in the design of the board and how well it works as a kiteboard, it definitely plays an important role and has a big effect on the feel of a board particularly on a wave
I have a 5'6 vanguard ( favourite kiteboard ever

) and a 6'1 bws/ dhd White noise- the White noise is Bigger wider and has more volume but the vanguard has way better low end because it has such a low drag design and it points so nicely upwind. I also have a 5'5 nugget which due to its width and volume coupled with the flat rocker beats the vanguard in absolute low end.
What I would be interested to see would be how big of a kite directional you could go for if you designed something purpose built to eat up the light winds.
I would imagine definitely a parallel rail/ vanguard design around 6ft x 22 wide or so with less than 40l volume and maybe a pulled in flyer/ stepped in tail with a lot of edge to still give the control. Then I'm out kiting super smooth light wind cross off indo waves with my 12m airush ultra ( sorry guys I just started daydreaming)
also plummet I definitely want to get into some hydrofoiling but I am still waiting a little to get into that as I don't have the time to put in right now