See this post for why the files get bigger.
I would keep the events for as long as you are editing these projects. After that, you can keep the original. You can always reimport it if you need it.
My suggestion is that you only want to import those 8mm tapes once, so you want to keep them.
Your video files are stored in the Events. The project file contains a text file that has the paths and file names and in and out points of the clips you use in the events. If you delete your events, your projects will no longer work.
Once you Share a completed project, you have a finished movie, so in theory, you could delete the event and project after that, but that would prevent you from ever using this footage again.
I suggest getting a large external disk drive and moving them there.
In the case of DV, it is already a full-frame codec, so the files do not get any bigger on import. But DV is plenty big, nonetheless.
You can SHARE the finished movies using SHARE/EXPORT MOVIE or SHARE/to iTUNES in Large or Medium size. This will be an h.264 file and will be much smaller. It will look quite good on playback. You will only find a problem if you have to uncompress it so you can edit it further.
H.264 is fine for playback of your finished movie. Only you can decide the value of your raw footage.