The largest single component of a starship is probably fuel tankage, followed by engineering [J-drive, M-drive, Power], followed by weapons in a warship.
The limitations of
1] only one spinal mount in a ship
- a] spinal mounts are integral to the ship's keel and cannot be replaced without scrapping the ship
- b] Meson Guns and Particle Accelerators max out at a given output; you can't just fit a bigger projection tube to a larger power plant and get a larger, more effective 'bang', there is a limit to what can be effectively fitted
2] Jump drives require 10% of the ship's hull per Jump rating number as fuel tankage
3] the manning requirements ['x' number of people to operate systems x 'y' number of tons for habitation x 'z' number of power points to operate life support]
tend to militate towards more hulls with more spinal mounts to fire on an enemy rather than One Great Big Hull. Seriously, what's the point of fielding a 1 million ton ship whose spinal mount is suited to a 500 kton ship? You could get 3 perfectly good 500 kton vessels for the price of the One Big One.