I think this one was not addressed yet. And while i don't -know- why FD did it like this, i remember when the people from wing commander saga explained why their ships were so unreasonably large. (Not that they feel too large in game. But people who extracted the models realized that they were much bigger than they would be to be expected. ) They first tried to use normal scale, but combat was basically just shooting at pixels, without ever having good visuals on the target. Only when they made ships much larger (up to 16 times larger!), combat turned out the way they wanted it to be.
So while i have no hard evidence to confirm that things happened the same way in ED, I very much think that ED would have the very same problem if ships would be scaled matching to their mass. Also i think if this happened, in terms of gameplay FD did the right thing by scaling them up without adjusting mass. Would the ships have mass according to their size, people would be all the rage about "unrealistically high" mass numbers. People have expectations how heavy a "fighter spaceship" should be and they have expectations on how it should look on the screen. They have no feeling that their expectations can't possibly match up. (At least i don't. Without the maths being done, i would have no chance to tell that something would be odd here. )
Sidewinder is about the same length as F-16 (14.9m vs. 15.09m), that sounds realistic to me.