No disk space on IMovie for large files.. but WHY?

I've read through several discussions already on how to free up storage space... But this does not explain why I am unable to move a large mov. file into Imovie to work on it... Essentially, I'm not copying the file to Imovie, correct? I'm just using Imovie as a tool to work on it... so I'm confused as to why I need "more disk space" to movie the file from my desktop into Imovie.... Here are the steps so far I've taken which have had little impact:

*Used CleanMyMac multiple times to clean up

*Deleted Iphoto pixs that are doubles and emptied the trash in Iphoto

So- a) Do I need to be overall making more disk space... (i.e. deleting more photos and movies)?

OR b)just deleting more space on Imovie specifically to make room for a 1G movie file?