So, this is an extreme example, but in Dawn of Rebellion, the Death Star is silhouette 20, making it very difficult for it to even hit a SUPER STAR DESTROYER!!! Even if you cap it at formidable, that still seems extreme, particularly when you realize that that would mean that a POINT-DEFENSE CANNON would be unable to hit a SUPER STAR DESTROYER (sorry for the caps, but YIKES [whoops did it again]). My tentative proposal is that we keep the same silhouette difficulty table, but we base it off the "preferred silhouette" of the weapon, I.E. a Point Defense Cannon has a "preferred Silhouette" of 2, allowing it to have an average roll against both starfighters and something like missiles (the big ones) (even though shooting down missiles isn't a feature in this game) while a Heavy Turbolaser would have a "Preferred Silhouette" of 7 (Hard against 5, easy against 9). I don't think this is necessarily the best option though as it requires making either a formula for applying to all weapons, or going through by hand and assigning a preferred silhouette to each one.
I think that because of their size and maneuverability silhouette 3 and 4 vehicles should still have the same difficulties, especially because their weapons are often fixed.
One alternative would be to just limit the silhouette of the firing ship to 8 regardless of the ship's actual size (although the difficulty would be the same for targeting it).