First, don’t ever try to compare the two NLEs at all, they are so vasty different, it won’t work.
PPro gives you a “project file” which is just that. A flat data-base file with info about the project, no media is involved.
FCPX give you a “Library” which is a vastly different animal. All media is (there’s an option to keep it external) inside of it, along with all cache files, etc. So there’s no “file” that balloons up, it is a “Library.bundle” technically speaking. You can (very carefully) right-click an FCPX Library and chose “Show Package Contents”. Just don’t mess with anything inside of there unless you absolutely know what you’re doing. But that is how to look inside the bundle.
You do have automatic backups of the project data in FCPX. Look in your Movies folder, for a folder called “Final Cut Backups.” These are what you’re more used to in PPro, they’re just time stamped backups of the data. You can revert back to any of these at any time by double-clicking one. It will open and be independent of your original. You could sync that folder to an online storage service if you wanted to. And like PPro, none of your media is included, so you’d want to back that up also, meaning, like any computer system that you make a living off of, it should have nightly back-ups running to a back-up drive, in case your drive itself fails.
If you wanted to, you could set the Library to store media in an external location, but you’re still using the same amount of drive space. Media inside or outside of the Library doesn’t save nor use any extra space, media files are media files.