Lol, I think we have different definitions of large ships. Your "cruiser" there is in the entry-level weight category my current game race would be torn between calling a corvette or a destroyer. Your battlecruiser would be comfortably considered a light cruiser. And if anything in other games I have tended towards labelling things in a heavier direction (the curent game is based somewhat on modern ship weights, other games have assumed that ships would tend towads getting heavier).
I see no reason there can't be a place for multiple ship types in the fleet. I tend to go with small ships for things I want lots of and want the risk of destruction dispersed for such as PD for instance whilst heavier ships bring heavier firepower. My current race has a specific design philosophy for each. The corvettes are typically between 1,000 and 4,000 tons and mount PD weapons. Destroyers are between 4,000 and 10,000 tons and are the baseline ship-to-ship combat units. Light Cruisers are generally between 8,000 and 12,000 tons and are like Destroyer+. They have the same protection as destroyers and the same weapons systems, but more of them so they bring more firepower and whatever extra combat ability I might want to throw in (eg. more fire controls or whatever). Cruisers are between 12,000 and 20,000 tons, they typically step up the size and power of the weapons systems as well as the protection from the light cuisers. When I get to them, heavy cruisers will be 20,000 to 30,000 tons and will be to cruisers what light cruisers are to destroyers and 30,000 tons and up will be battleships, again a step up in terms of offensive and defensive capability. I haven't bothered with fighters as a general combat approach because its still on a patch that interrupts every time a fighter is built. Sensor ships are usually based on light cruisers with all their weapons stripped, replaced with a powerful sensor and a hanger bay containing 4 - 6 sensor fighters.
To be fair, this is more based on RP than raw design efficiency, but I think having a range of ships is useful so you can decide whether you want more, cheaper ships or less, more powerful ships or a mix in between.
In my campaigns I tend to make the ships for their roles and the sizes tend to grow with time. So a destroyer that is 8000t in the early game might grow to become a 15-20.000t ship later on. The main thing is that their role in the fleet remain the same.
In a developed game then my fleet sizes and roles usually look like this...
Corvette 2000-4000 tonsThese are specialized patrol, scout ships. They have rudimentary weapon systems if any at all, they are not meant to go into actual battle. Their means of defence are usually stealth and/or speed, depends on what they designed for.
Frigattes 6000-10.000 tonsThey are either old destroyers or newer ships tasked with the role of escort for commercial ships or other slower ships. Their main armament is that of defence and especially point defence to defend support ships from missile attacks, but also some beam weapons for dissuading an enemy to close in.
Destroyer 15.000-20.000 tonsDestroyer roles is both fleet escort but also scouting, patrolling and engaging enemy ships at limited capacity. A destroyer are suppose to be able to act on their own or in smaller groups, depending on their mission, but destroyers are very rarely deployed to a system alone but in small flotilla forces. They need to have both offensive and defensive weapons with a focus on the defensive side and these ships tend to be the most numerous as single type of vessels in a fleet if you don't count FAC/Fighter crafts.
Cruiser more than 20.000 tonsThis is a really large ship whose role are basically long range patrol, reconnaissance and projection of power. They are suppose to act both alone or in a small task-group of cruisers. They are basically very big heavily armed and armoured scout ships and have basically all types of systems included in addition to a decent hangar space for a plethora of parasites depending on their mission type.
Battle Carrier more than 20.000 tonsThis is an armed and armoured ships with the main role of carrying strike crafts for its main offensive capacity, usually in the form of missile strike crafts. The ships always have some for of anti-missile defences and a strong beam defence. They are suppose to hit and destroy enemies from a distance but they also can engage in close beam combat action such as jump gates or defending a planet. These ships would be the main capital ship of any fleet and would be like marrying a battleship with a carrier.
There might also be allot of other types of ships with different roles such as light carriers, assault carriers, jump tenders, and many more...
But I think that size is generally a definition of how advanced and developed the economy and technology has become. Bigger ships might take more time to build but they can have quite effective systems and allot more staying power. Smaller ships are more versatile and can be spread out over a larger area and obviously does not sting as much of you do happen to loose them. But in general if you can afford three 20.000 ton ships to do a task that is generally more effective than six 10.000t ships doing the same thing as the components on the larger ship probably are more efficient and the larger ships are more resilient in general. On the other hand six ships can be in six different locations where three ships can only be in three... so it is a give and take.
When it comes to offensive capacity then small ships can be built allot faster than the same tonnage of large ships... so if you want 30.000 ton of raw offensive power (say missile strike crafts) it is far quicker to build 30 1000t FAC than one 30.000 ton missile cruiser and the smaller ships are more efficient for doing overwhelming quick and surgical strikes. The big ship is more about presence, taking and holding space far way.
If all you need is defending yourself then concentrating on smaller ships is usually far more efficient... but if you want to project power over longer distances then the bigger you can make the ships the better in general.
But... at the same time... you should always try to maintain a good balance between different types of ships... so, small or large are both highly valuable no mater what.
