If you only look at the movies, the object in RotS and the Death Star I are, diagetically, completely different objects. The proportions of the RotS object don't match those of the Death Star. Unfortunately, behind-the-scenes comments by George Lucas and post-RotS Expanded Universe material indicate it was the Death Star*. In order to resolve this conundrum, you have to ignore the visual effects, ignore the EU and GL, or assume that it was taken apart and reconstructed at some point.

The standard online reference for the Death Stars is here.

* Or maybe the EU has decided that it was the Death Star prototype, later relocated to the Maw and repeatedly retrofitted by Bevel Lemelisk and Qwi Xux. I really don't know, or care that much.