Why does iMovie consume free disk space on launch

I have a few movies in iMovie that Ive created from QT screencasts and audio recordings. I have about 8.5GB of free space on my mac, according to Disk Utility upon launch. All is well until I open iMovie and if I check back with Disk Utility, it goes down to 7, 6,5,4,3,2,1,0 until my computer freezes and i must shutdown forcefully.

Any ideas?