Also, it's worth pointing that Kraken P, BMM and Hull have different purposes and can't be compared solely on the basis of cargo capacity.
Kraken P is a space station with landing pads and shops, where you can trade in space, on orbit, or on planet surface. To me Kraken P is akin to a mobile supermarket with parking lot, where you can not only take customer vehicles, but also deliveries, so with just in time deliveries you may not need to hold much cargo: get batch delivered -> sell -> re-order. I would note, though, that cargo hold for Kraken indeed seems way too small.
BMM, in my view, is a mobile boutique for more exquisite merchandise, as well as a mobile office for sole trader type business negotiations (for large corporates there is 890). BMM purpose is likely to be deployed only when landed. You won't have any fast moving consumer goods for on-sell purpose deliveries, and therefore you may require a larger cargo hold. You also likely won't be able to trade in space.
Hull series, in my opinion, has nothing to do with shops, rather those are freight industry. Hull E is something like an IRL marine supertanker able to carry 3+ million tons of oil (or other cargo), or like a super container ship. Hull D is closer to a large tanker or cargo/container ship. Hull C is like a cargo train, that picks cargo from a sea-port (where hulls E and D are unloading) and deliver to freight terminals. Hull B is like a large freight truck, that picks cargo from a freight terminal and delivers to a smaller terminal or a large warehouse. Hull A is like a small truck, that delivers cargo to retail clients. There is nothing more to it: no trading, no landing pads, and for most, no planetary landing with cargo. Just freight. Large cargo holds make sense to me.
So, comparing Kraken P and Hull E is like comparing a supermarket to a supertanker and saying that a supermarket needs to be able to store millins of tons of goods, because it has the "super" in it's name.
