Of course we have to remember that there need not be a correlation between what a volume of ship-internal space can hold as 'cargo', and what it can hold as anything else. With cargo canisters all being the same size, I'd guess that one containing a tonne of gold would be practically empty, volume-wise, whilst one containing something considerably less dense would be filled to the 'brim'. On which basis it's probably fair to say a standard canister is built to be big enough to hold the least dense commodity. Which means the
volume of a cargo-hold of (say) 4 tonnes is likely to be
way bigger than an SRV hanger of (say) 18 tonnes.
Or not
