Why is extra space taken up when importing video?

I am considering using iPhoto to manage all my video files from my iPhone / Flip video recorder.

I imported 18 GB of video files into iPhoto, and I noticed the iPhoto library size increased not just by 18 GB, but by 36 GB, an extra 18 GB.

Does anyone know why importing video would require twice the space?

When I click on any individual video file and export it from iPhoto, it is the exact same original size. I would not think that a thumbnail would take up such space. Any ideas?

Thanks,

-Lee