Let's also not forget that Palpatine has to build this DS in absolute secrecy, since he is still relying on the Senate to help maintain the peace. After ANH, the secret is out, the Senate has been disbanded, and the Empire is finally in full swing. Palps was able to throw all his resources into the second Death Star (which wasn't completed by ROTJ, I might add. They had only focused on finishing what needed to be finished, to lure the Rebellion into attacking.)