Title say all I guess. Seems if say a module is 32 tons and I have 32 tons of cargo space I should be able to move it myself.
No, because a cargo hold is not simply empty space into which you can fit whatever you wish. Your 32 tonne module is a weirdly-shaped thingamabob, which fits nicely into the space made for it in your ship but is not easily broken down into 1m x 1m x 2m cargo cylinders for stowing onto a cargo rack.
And even if it did let you break down modules into bite-sized chunks for transport, what will you do with your module if a pirate comes along, hatch-breaks you and steals 1 tonne of your module? What will you do with the remaining 31 pieces of a now-presumably-useless-without-the-missing-piece module?