It could be made more solid through and through, thus be less likely to be crushed at lower depths, thus become smaller. Most of the space is probably due to life support and the driver being able to move around in it, and not to store the goods.

In the VBS.tv piece mentioned before, a guy that purportedly was involved in making and presenting a kind of towed torpedo to the cartels, said he wouldn't be surprised they'd have such a remote controlled device already, and that he was sure they we're already looking into it. You can watch it.

And I remember seeing something in the news, not long ago, that they we're already doing it for shipping cocaine directly to Europe.