Oil Ocean is Eggman's main base in the overworld. He does keep the size of it about the same, though he visions transforming it into his childhood dream, Eggmanland. Sonic tends to limit this though, and Eggman's not yet aware of Sonic's issues so he has been thinking of more sneaky ways to expand his empire rather than head right into a head on attempt. Oil Ocean remains guarded by his inventions, whether he's there or not. It serves him great importance. The temperature there and the lack of rain makes conditions almost desert-like, but that will not hold Eggman back from achieving his goals.