Sorry, but I couldn't help but ask:
Is the sequence (what you cut together) 10 minutes long, or all clips combined (as your source)?
If it's the latter, I don't have an answer.
If it's the former, how long are all individual clips?
Have you opened the clips in the iMovie scratch folder with Quick Time and looked at the information inspector (Cmd + I) to see more details about your clips?
If you use DV PAL as a setting every hour of footage will take up to 12 GB of storage, or 3.25 MB/s.
And PAL is SD (Standard Definition) and not HD.
Aia:
Both projects (and especially the first one) consist of quite a lot of short clips...I can only suspect that each clip must somehow be retaining information from its entire source footage- could that be the problem, and if so how is that happening? And if not, what is happening? And how do I stop it from happening?
What do you mean with short clips? Have you imported the footage via camera or via hard disk or what?
A clip imported should have no remaining information that links to longer footage. Only if you make a subclip via iMovie (if that is possible - a subclip is a shorter clip made out of the bigger one, but with no extra associated media files, it just points to the original bigger file)
a clip can be shorter but still point to information that is bigger/longer than what you see.
So in the end, could you just give us a rundown of how you created the project?
Where's the footage from, how did you import, how long are the individual clips?
Because the length of a sequence doesn't define the size of a project, the media files are.
For example: at work we have a big project for a stupid candid camera show with 1300 tapes of digitized video which takes up several terabytes, but the following sequences are only about 23 minutes in length and amount therefore only to about 40 GB of final data.
Sorry for the long post.