OK..I'm preplexed here.
I imported all of my iphone videos ( which were stored in iPhoto) into iMovie.
But, I was surprised to see that iMovie only copied the videos, they did not move them. Thus, i had two copies of the videos.
BUT, when i went to make sure that I did in fact have 2 different copies ( and not just a referenced file) i found that not only did I indeed have 2 copies of the file, but the imovie version of the clip was roughly 2.3 times the size of the original clip, which was still stored in the iPhoto package.
The over doublling of the size of each imported clip was found in every imported clip! Meanwhile, the quality of both clips, opened in quicktime, look pretty good to me.
What's going on???
iMovie may have converted it to a different format, such as H.264. Locate one of the iPhoto videos in Finder, then open it with QuickTime (either QT X or QT 7). In the QT menu, select Window>Show Movie Inspector - what does it show against Format (including size)?
Do the same with the Event in iMovie. In Finder, go to your iMovie Events folder and open one of the Events imported from iPhoto. All the clips in the Event will probably be labelled with the .mov extension. Open a clip in QuickTime and check the format and size.
When highly compressed AVCHD clips are imported to iMovie they are converted to Apple Intermediate Codec (AIC). This effectively decompresses the clips, converting them to full frames for ease of editing. AIC uses about 40 GB per hour - about 4 times the size of AVCHD (.mts files).
Of course, your iPhone clips in iPhoto may already be H.264 (don't have an iPhone yet - hanging out for iPhone 5). But check the format for both the Event clips and the iPhoto clips - I may be way off-beam here!