RE: Actual volume of Isonics
To add my reflections and (small) experience to Ian's answer.
Even when considering the very lower wind range end of a board, volume is not all, but rather "available volume" is. That is: remember that if part of a board's volume is not under water, then it is not contributing to buoyancy and it behaves the same as if it was not there at all. The center part of a much domed deck may not be contributing. The nose of an high-rise-rockerline board may not be contributing. As a result, a wide, thinnish, low nose rocker board - like an iSonic - must see almost all of its volume contributing to buoyancy when standing still and submerged. I have no iSonic experience, but I reckon that my (short, wide, straight, thin-nish) Evo 83 feels better (bigger) at schlogging than my previous 90 (stated) liters '00 RRD WaveCult 260. Once on the go, its real lesser volume makes it feel better again.
In the end, real phisical dimensions should be taken with some reasoning, rather than just taking them as they are. Just like we all know that a 5.8 race sails wind range is much different from a 5.8 freestyle sail's.