What application are you importing your video into? iMovie? Final Cut? Depending on the original file format, it may have been automatically changed to one that the application can read - in that case, the file might have gotten larger. For instance, a song in MP3 format is quite small; if you convert it to AIFF format, it suddenly balloons to 40+ MB. The same thing may be happening with video.
Second, as denthed has mentioned, you do not have enough space on your hard drive. If you want to edit/render/burn video, you will need at least as much empty space as the combined size of all your files is in order for the computer to be able to work with the video reasonably efficiently. During my last project, I amassed a total of 130 GB of various files and made sure I had at least that much available - personally, I prefer more, so I had over 200 GB empty. When I was done, I deleted all the files and simply moved a .dmg of the movie to an external for possible future burning.
If you do not work with video, your OS still needs an absolute minimum of 10 - 15 GB of empty space at all times in order to run.
Sounds like maybe you have imported duplicate movie files to the iMac or not emptied the trash..?
Use > OmniDiskSweeper to locate and delete any duplicate files.