colas said... if the take off zone is quite predictable, you can get away with sub-7' boards. Note that sub-7' are really interesting in very small waves, on chest high waves and more, no problem to move a 7'6" around.
On the thickness, note the the quoted thickness is at the stringer, it is the thickness of the rails only for flat decks. Domed decks can bring you surprisingly low-volume rails while keeping a comfortable volume
I totally agree and sums up my conclusion:
a) 1x total weight to volume ratio can be done and wave performace is very enjoyable if you got the skill and stamina
b) giving the same volume: the larger the surf, the longer the board.
@Kami and Colas: the 6'10 gong totally makes sense in chest high surf or less, its volume means it can be ridden by up to a 90-95kg supper. Rails wise, it is true you can get a very domed deck to get thinner rails to a certain point or think outside the box with the shape.
Just how much a sub 7 vrs a 7+ at same volume overlap on chest to head high surf is a matter of preference I think.
Probably 2yrs ago, somebody had the same conclusion but volume ratio was 1.2 and shortSUP was sub 8, therefore everything written above could be discarded in the near future.