Why does rendering use so much space?

There are a lot of things I like about FCPX as far as it's time consumption, background rendering and what not. However, the render process specifically seems to chew up a lot more space than it did before and I cannot get around this because FCPX automatically renders in the background. I obviously want to render my video so I can view it in full quality, but is there another way that makes it use less space on the hard drive? I feel like the project (after rendering) occupies more space on my hard drive than even the raw footage sometimes. I have to keep manually pausing the renders or my hard drive will run out of space before I've even done most of the editing.

If I would start running low before I could just access the files and delete some of the old render files, but I don't know how to access them how it's set up now. It's just a library.fcpbundle.

Any suggestions. I have multiple hard drives with a lot of space available, but I would like to use them as economical as possible to get the most out of them.

Thanks for any advice.