These kind of topics baffle me. You may care to read Jules Verne's 'From Earth to Moon' on the question of armour and weapons outperforming each other as the generations go by.
The point that I feel should be made (and has already been made) is that the notion of 'realism' when talking about how weapons and ships in the far future will work is vacuous. In fact, I'd go so far as to say that 'realism' and 'science fiction' aren't really the same kind of thing.
During the Falklands war, a plane carrying a single Exocet was able to destroy a troop ship, although it could have itself been shot down by machine gun fire. As someone else pointed out, go back a hundred years and the situation was very different. Go back a thousand years, and the only way a vessel could sink another was by ramming, and boarding was a much more common practice in naval warfare.
Who can say what the far future will hold? Maybe capital ships are capital ships because they can continue in service for years without undergoing major repairs, whereas a small fighter might require an almost complete refit after each sortie - rather like a Formula 1 car after a major race. This wouldn't interest the EV/O player much, but would be very important for governments commissioning ships for protracted campaigns.
On the other hand, maybe what makes a capital ship is the fact that it has so many crew, so that it can effectively capture other ships once it has disabled them.
Or, again, maybe a warship has got a lot anti-personnel armour and weapons, so when it lands on a planet it can subdue a civilian population or military base.
Or, as another option, maybe the problem is training enough pilots - as in the Battle of Britain. If the same pilot could fly a battle cruiser or a fighter, but has a life expectancy of three missions in a fighter and a hundred in a battle cruiser, it may (for a government) only make sense to build capital ships. For a mercenary who doesn't want to split his profits and always believes his luck will hold (and can't afford a big ship) a small fighter might seem the right option..
Or, how about if the way technology develops means that flying and blowing things up is fairly simple, but transmitting a message through deep space requires a massive installation. A capital ship would have to be big enough to carry this equipment, and well armoured enough to give it enough time to get its message through no matter what the attack. In some kinds of historical wars, intelligence has been vastly more important than any other kind of military action.
Or, again, maybe in your SF universe sending a ship through hyperspace creates some kind of distortion field, no matter how big the ship is. Sure, sending a big ship through takes longer (maybe three days) but there's a maximum on the number of separate Hyperspace events. So, obviously, if your choice is say, a maximum of 36 ships (I wonder where such a figure could come from), you would want to send 36 capital ships which might themselves carry non-hyper capable smaller ships. For defence, on the other hand, you might want to be able to keep sending up smaller ships.
Or it could be the other way round, where an attack would primarily be smaller ships, because of the effects of mass on Hyperspace travel, while for defence you would put up enormous ships which could stay in space indefinitely, rather than those small, weedy ships which have to be refueled every few hours...
When you are making your own reality, then realism is something you define yourself...
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M A R T I N • T U R N E R
Go me!
