As Sigg points out, you can fit HG fuel in there. The 4 dton of fuel would drop you down to 2 dton left over, which is right at the cargo it is supposed to have. (I forget the number; it is 1-3 dton.)
While I had always assumed the left over 10 dton were occupied by the "vast databanks" that were described in the color text, but never specified in the design.
However, since the whole "vast databanks" thing is a massive handwave anyway, I don't see anything wrong with handwaving them away with a custom bridge.
Since the purpose-built X-boat doesn't have a maneuver drive, it doesn't need any of the avionics, sensors, and other stuff. It can also be assumed that the "slop" stuff assumed in the bridge tonnage (airlocks, lifesupport, ship's locker, etc.) is minimized. This would let you say the databanks are taking up the saved space.
For game effect, just say that the X-boat bridge is the same size, has improved commo gear, can store XXXX amount of data, but can't be used with a manuever drive. It probably costs more, too. The cool thing about this is that the bridge would then also be available to bases to use, which also don't have maneuver drives, but could use the improved commo and data storage.
As for the bigger question of power plant fuel usage, I am somewhat undecided. I can see advantages to all three solutions. (The three solutions being 1) use HG forumla, 2) eliminate PP fuel requirements, 3) use 1 or 2 dton * PP module size.)
