Was the entire planet the Garden of Eden? If not, what what was the purpose of the rest of the planet?

As I understand it, when Adam and Eve committed the Original Sin, God cast them out from the Garden of Eden to live in the planet we have now. To cast someone out, means there must be 2 places, here and there. This gives me the impression of the Garden being a separate place from current Earth. If that is the case, why make a planet as big as ours is (in relation to its then-current inhabitants), only to focus on a Garden? Why not make a whole perfect planet? If the entire earth was the Garden, how does one get cast out of it? Is it just a fancy way of saying "So God made the planet kind of sucks in comparison"? That is my opinion, which I have shown some reasoning for. However, I seek to know if the Bible's account can be shown to support this, or if it does not allow for any such idea.