It certainly raises questions, ones that the EU tried to answer in a myriad of ways. According to current EU, the Mon Calamari were in the Republic and later rebelled after the Empire decided to take direct political control of their planet for reasons. So this made them go from pacifist to fully thrown in on the war effort. Those cruisers were originally supposed to be civilian ships but were later converted into multi-role warships. The Empire should have been able to bombard their homeworld and destroy all industry so it's not exactly clear where those ships are coming from. Did they already have hundreds in service and after their homeworld was attacked they all threw in with the rebellion? Where did they do the refits? What are they doing for spare parts?K. A. Pital wrote: ↑2018-01-22 07:12am It was pretty clear even the Empire did not control the whole galaxy.
Otherwise where from did the Rebels get these 3km long Mon Cal ships??
Probably the best answer, one not supported in any of the material I know of, is that the Empire is more of a core thing and there's a bunch of polities rimward that never felt comfortable joining up with the whole Republic thing and are now leery of the Empire trying to make them join the central government by force. The Empire is trying to avoid giving them a reason to form an alliance against it and so tries to play nice even as they suspect some of them are funneling resources to the rebels. The goal of those outside polities would be to weaken the empire as much as possible while not helping the rebels enough to gain a victory because a new republic might feel expansionist again.
In so far as depicted in the films and the original EU, there wasn't really any concept of "rebel-held territory," planets so secure that the Imperials could not afford to project power there for fear of a publishing fight. Any place the rebels decided to place a fixed base, the Imperials could attack. Yavin, Hoth. So we see the Rebel fleet at the end of Empire and are thus back to the question of where they resupply from. It's unclear whether the fleet could "live off the land" as it were, growing food in agripods, mining resources from asteroids, manufacturing replacement parts, getting hypermatter from wherever it comes from, etc.