The EX1 files remain at their native 35mbps rate thus taking up a lot less space. The HMC files must be transcoded to Prores. These Proress files are more along the lines of 100+mbps. Until Final Cut can natively edit AVCHD its going to require larger files. I have FCP7 and have been testing out the Prores LT option. It seems to decrease the size by 25%. We are only running at 1280×720 so the extra compression isn’t noticeable.
The issue here is you can’t transcode one heavily compressed format (AVCHD) to another heavily compressed format without serious quality loss. Thus you have to work in Prores until you finish to keep the macro-blocking and compression artifacts to a minimum.