No offense to anyone that don't agree with me but I just don't understand why (for logical or logistical reasons) you would designate a name for a weight of a ship rather than the role it serves. Calling any ship between 6-10000t a destroyer does not say a thing about it's purpose and the tonnage is there to show its size anyway. I'm not saying its wrong or anything, I just don't get why it's practical to do so other than confirming a sort of level system.
There is also the tendency that the game favor bigger ships as technology rises as it speed up production, lower production cost/time on refits and so on.
In one game I had a cruiser at 45000t escorted by 15000t frigates while destroyers where 9-12000 tones. The difference in the ship where its intended role, deployment time and armament layout. The frigates where mainly for protecting something else and large enough to accommodate all the weaponry and auxiliary vessels to do their job. The destroyers were built around the concept of squadron warfare and for self sustainability in that role or as a picket or screen for a bigger cruiser task-group or perhaps a reconnaissance in force where stealth is more important than brute force of a cruiser. In this game I also had very small frigates at about 3-5000t that were just beam or missile armed escort ships. Basically just one type of weapon system and not able to operate alone. The reason I call them a frigate and not a patrol ship, corvette or anything like that was because they have a long deployment time and are suppose to operate with a fleet on long missions (usually above three months). Any ship with a deployment of three months or less would be a patrol ship, corvette or something of that sort no matter its size. Obviously there are no real reason to build such ship larger than maybe 2-6000t but there might be.
The reasons why I decided on these designation are because of the historical background of different ships. A frigate was basically the smallest military ship equipped for long voyages and able to (cruise) perform more or less independent action. The ship of the Line was the battleship of its age while the frigate was the cruiser of its age. A ship of the line was a large ship that would perform the majority of military battles together with or without frigates. So the ship of the line became armoured ships became armoured frigates and then battleships. In WWII a battleship is mainly equivalent of a heavily armoured cruiser and actually they were not all that much bigger for the most part to a very big cruiser just heavier and also wider so they would not sink...
Cruiser was in the beginning a smaller warship intended for longer missions while "battleships" was only meant for shorter more decisive action. Cruiser was more a type of ship than a real classification and could be a sloop, corvette or a frigate. Cruiser were later changed into the armoured cruiser and then into the type of ship they were in WWII.
So in my world a battleship is just a cruiser with a smeg load of armour and shields as a significant part of their total size. Cruisers have weaker armour but can certainly be as large of even as heavy, it all depends in technology level in my opinion what is what and on what other ship you have in your fleet.
I could even declassify a ship from battleship to cruiser if I no longer deem a ship to be an actual battleship in comparison with newer ships or changes in the fleet structure. In practical terms there is no difference in the role of a cruiser or battleship. A carrier on the other hand is just a ship that can be of any size that just has the majority of its space dedicated to hangars in some capacity. In Aurora though the difference between a large cruiser and a carrier can become a grey area so I also use the term battle carrier which basically is a mix between a carrier and a cruiser, size still have no real meaning here.
I'm trying to somehow relate to what I think would be a realistic interpretation of how humanity would go about naming ship classes in space in reality. Today for example the term cruiser is almost an extinct type of ship in place of frigates and destroyers. Even the term frigates are starting to shift because today the only difference between a frigate and destroyer are it's size and when that happens the classifications tend to shift and change. Most really modern frigates are becoming more short ranged coastal or amphibious or recon multipurpose ships while destroyers are the escorts for other ships with both defensive and offensive capacity in many varied sizes.
So, basically, my interpretation of the difference between a destroyer and cruiser in space would be that cruisers would carry larger capacity fuel, crew and auxiliary vessels for truly independent long range action while destroyers would always act with other destroyers and intended for shorter missions and closer to their base of operation. That is also why cruisers would naturally be much bigger than a destroyer and why you would ever need to build one over the other because they serve totally different roles.
In my opinion there are no real reason to build large multipurpose ships (other than for fun) if you only control a couple of systems. But by the time you control several real sectors and your empire span over a very large area you can really benefit from being able to deploy larger long range multipurpose ships. These ships can only be build and maintained at very few places in your empire but there is other benefits that outweigh those drawbacks.
There is obviously nothing wrong with classify ships by weight... it's being done today as we speak in our own navies. Even if there is a tendency to shift classification to role rather than size over time.