A Small Ship Traveller Universe - A Bit of Handwavium to explain it.
In Mongoose Traveller, Capital ships (i.e. those over 2k dTons) are required to be in section. The largest one that can be done in just 2 sections is 7,500 dTons, which, as it happens in MTU, is the largest volume that a TL 15 anti-gravity field can support. Jump drives need A/G fields to make jump bubbles, so that also limits how big a jump capable ship can be. A/G fields affecting volumes of space, not mass, also explains why space ships are sized in dTons and the mass of the ship doesn't matter or affect how many G's acceleration it can pull. Due to the rather massive drop in both size and cost of spinal mounts, even meson guns up to Type C (the largest being D) could be mounted in a TL 15 craft.
This was first posted to the TML, but this is the "official" blog of My Traveller Universe, so I'm putting it here so it is made official and stuff.