I'm fairly sure Apple assumes most people want to keep their original footage... "forever." Media management in FCPX has received quite a lot of praise from professionals.
Everything comes down to prep and organization.
Projects are just pointers into media clips and render files. If you want to trim down your projects, go to the File menu and select Delete Project Render Files...
Deleting Event clips will kill a project no matter what. Archiving a project without the associated Event media is pointless. Best advice is to always keep your original media. That way you can reconstruct an Event later for the clips your project requires. As far as I know, there is no way to delete (to the trash) Event clip ranges.
If you know that you won't be using 90% of your footage and want to delete the extraneous original footage, I would recommend using Quicktime 7 Pro and selecting the ranges you think you'll need beforehand. This is easily accomplished by opening your original clips, setting an In and Out point (just like in FCPX) -- select Copy, then File > New and Paste. Save as a Self-Contained clip and import it into FCPX. QT7Pro does not alter the original footage (as much as possible, I imagine it has to occasionally build an intitial i-frame if your original is something like H.264, but other than that, you can't tell the difference.)
To preserve a project entirely, you will need to back up the associated Event clips as well. Those are the clips you need to worry about trimming. You should use Copy files to Final Cut Events folder on Import. You should be able to delete any transcoded media (High Quality and/or Proxy) before archiving (don't take my word as an absolute on that -- Tom probably knows for sure.) For simplicity, you should create an Event for JUST the clips you will be using for the project to backed up. That way, at the end, you can group the Event folder and the Project together for archiving (and with the smallest footprint).
You could also simply Export your (trimmed) project as H.264 (FCPX uses the highest quality settings) without titles, effects, generators or transitions to retain the portions of clips you want to save/archive and delete all the original media — and forego everything stated above. You will still have an exceptional quality source for any subsequent projects using the same material and it would be compact enough. I wouldn't archive with ProRes (except maybe Proxy... it's about the same bitrate as the H.264 generated, except I think the H.264 would still be a better choice.) Using ProRes kind of defeats the purpose of archiving.