The problem with ships having realistic proportions relative to each other, is that fighters and corvettes become less then specks when you are viewing a capital ship. The star swarm demo thing shows this pretty well. So i think fighters will always be pretty large compared to bigger ships just for ease of viewing.
Ok, but it depends on other features.
First, I am not talking about perfect scale, because we do not have "real" spaceships to use as comparison. However, for the purposes of immersion designers could aim at making the proportion as best as possible.
Second, the problem with the viewing depends on the controls we will have during ship battles. The mechanics and the tools should dictate what the “correct” difference between sizes is. I do not want proportionate scale on the game map. In the map, fighters can be as big as planets and that is part of the game.
Third this obviously depends on the player, I am a perfectionist and making ships with coherent sizes is sometimes more of a compulsion 
Battleships and dreadnaughts feel more powerful when the fighters and corvettes are so small and frail compared to them.
Although you can compare the ships by making one of them a part, it would be nice if this was an “official” feature. I am no programmer, but I do not think this is difficult.
PS: I lack the appropriate knowledge of the English language to describe how excited I am with carriers 
