For a rectangular homogenous box in water, chopping off 10 % of the mass leads to the same percentage reduction of volume (boyancy) and surface area in contact with water, regardless of whether you cut the longest or the shortest (end) side.
Doesn't it?

For this discussion it seems important to disinguish between reducing weight by reducing hull volume from reducing weight but keeping hull volume (i.e. reducing specific weight of hulls).

I totally agree, however, that often statements of hull length are assumptions without data. It is quite common to think of hull length in association with sailing in displacement mode, i.e. longer hulls = faster speed. For a 20 foot non-planing keelboat (monohull) the theoretical max speed is ca 6 knots. Beach cats easily sail much faster than that, hence it is obvious that we are often not in a pure displacement mode. We do not need the long hulls to have fun.

However, going from a real short boat to 5.75 improves behavior in waves and reduces pitch-poling susceptibility immensely.

And it is of utmost relevance to the overall performance where the volume and weight is placed.

Stein