While I will freely admit to not being any kind of suppressor design expert, the length could have to do with the number/spacing of baffles. While internal volume of a suppressor is going to be one component of the noise suppression capability of a given design, the number of baffles may well play into it as well... otherwise, those make-shift oil filter suppressors (er, solvent traps) would offer some absolutely stellar suppression numbers, as the internal volume of an oil filter could easily be quite large (comparatively).
Or I could be full of shit... you decide.