And note that if the Empire were to use peer-level ships they would be subject to attrition. The rebels would jump in, ambush one ship with a barrage of fire a

And note that if the Empire were to use peer-level ships they would be subject to attrition. The rebels would jump in, ambush one ship with a barrage of fire and jump back out. This can only be defended against by either making the ships powerful enough to pretty much preclude this, or by packs staying close enough together. And if you work with packs you get a certain amount of flexibility but at a higher cost to accomplish the same thing. There is also the fact that they are fighting with shields and energy weapons. Shields mean the first damage a unit takes doesn't happen, energy weapons mean no heavy hits from small units. An ISD comes through unscathed when attacked by reasonably light forces and can't meaningfully be ambushed. That's a major psychological factor.