Thing is, a lot of those weapons are produced in space. And that means it's a lot easier than producing it on the ground. Compare the WWII-era German Schwerer Gustav artillery, and then the Hellbore Cannon.
And I doubt ground-based artillery would be able to even compare to space-based batteries. The Schwerer Gustav was the absolute limit of artillery, and it was past practicality.
Resources? Asteroid fields - there's little that they don't have AFAIK. Iron, Infernium, water, and a lot more stuff. The only thing that would need to be done on planets on a large scale would be farming. And once again, open fields. It doesn't matter if you blow up a few hundred kilograms of grain, so long the enemy does not secure it. Scorched Earth FTW.