The Classic Ships wiki page wrote:With one or two exceptions all the ships of this time are pure spacecraft, built solely to fly between Star Systems, with the GalCop stations that orbit all inhabited Worlds as their departure and arrival points. As a consequence they have no atmospheric or planetary-landing capability as these are simply not needed. They are therefore built in space at huge orbital shipyards which are dotted around the Eight at suitable planets and moons.
Due to the difficulties of working in zero-G, the need was for designs that can be fabricated relatively quickly and easily. It therefore soon became clear that the optimum manufacturing method was to design and build large, un-sophisticated hulls, with huge panels of Duralium Alloy being welded together into simple, easy-to-construct forms. The Duralium panels used can be as large as five by three metres and are nearly 1/2 metre thick, as this offers maximum protection against micro-meteorites and radiation, and also some protection against laser fire.
By building the ships as large as they are, it is also possible to include all the equipment, running gear, engines, fuel tanks, holdspace, living quarters, life support and ordnance within the hull itself. This offers maximum protection to all the ship's systems.
The size of the ship is not the only limiting factor where equipment is concerned. My opinion is that it is very likely that some of the equipment items themselves are vast and so even a 'big' small ship may not physically have the available space for some items. I also think that older models in particular were simply not designed to have more recent equipment fitted so the control and power infrastructure is absent and difficult/impossible to retrofit. Older ships also date from safer and more peaceful times, so the fitment of Military grade equipment was also not considered necessary when they were designed, and so again the associated power and mountings etc are not in place.