|Close enough to a displacement ton. Original|
|Official US Navy Photo, Original|
Now, let's talk about commercial servers. Rack Units support servers and other high-end computers, and can take up a fair bit of space. The standard one is 6'(180 cm) tall, 19" (48.26 cm) wide, and 36" (91.44 cm) deep. That gives a volume of about 0.8 cubic meters. A bit small, but add a few together, the connecting cables, power equipment, and soon there's enough for a displacement ton. Looking at those specs, the computers used for other purposes on ship are about a standard enclosed Rack Unit in size. Computers in space are not small things.
A shipboard computer is doing a lot more than my current Windows 10 laptop, and when running normally procured software, is bulletproof in reliability. It's a computer that is running mission critical programs realtime programs with minimal failures under normal conditions. It's also operating in all sorts of unpleasant conditions for a computer, where it can get jolted around in bad weather during atmospheric operations, spaceframe lightning strikes, before it gets into weapons effects. While with the appropriate software and libraries, you could run it off a commercial personal machine, but a ship is best run on a toughened system. Also, there are requirements for heat removal and power distribution. Most personal computers are designed for operations in ambient temperatures, and users do not notice the heat unless it causes issues. The ThinkPads used on the ISS require extra heat removal. Mind you, when artificial gravity works, normal thermal currents will remove the heat from personal gear, but the big iron will always need forced cooling. The way I see it, a shipboard computer is going to consist of multiple, redundant CPU's running the same task and 'voting' on the most correct solution, and toughed against 'normal' abuse. Starting adding in cables, ventilation, and power supplies, the space adds up quickly. These systems are abstractions, not gospel.
Now, why are more advanced computers taking up more space? Well, as I see it, many of them are shipboard server farms at introductory tech levels. Also, it could be an exotic technology that requires extra space to maintain states needed. Some people have recommended requiring archaic technology in starship computers as they may not work in Jumpspace, or it makes it eaiser to repair on low tech worlds out on the fringes. I prefer a simpler approach. Higher model numbers are, more or less, server farms aboard the ship, with the ability to switch from 'role' to 'supercomputer' very quickly. A model 6 computer can run several intercept programs, run an active countermeasure program, and control the ship's M-Drive at the same time. This is non trivial work that a modern system requires a separate machine, plus control cabinets spread through out the vehicle in use. Again, we get into ergonomics of equipment, and at some point, padding the size just so components can be handled and not lost is going to occur. The nature of Jump Travel alone is going to require extensive computer work. I am not that keen on exotic physics, but I think of Navigation programming as reducing frightfully complex mathematics that make the most students wilt, to making it possible for almost anyone willing to learn to astrogate.
Generally, environmental requirements, history, and reliability, including maintenance, needs are going to drive computer size as much as anything related to hardware performance. The space for power supplies and cooling requirements is much under appreciated by fandom. As a relic of the fiction that set up the game, large computers make sense, as well. So, just let the large scale computers continue to exist. Perhaps the farm of quantum computer running a starship require constant gravity, specialized power, and a high degree of shielding to keep operating.