Importing video

I import a video file that is a about 700MB, but when it is imported Power Producer is telling me it is taking up about 7,000MB. I don't understand the discrepancy and I assume it's why when I go to burn it's not giving me the option, only the option to create a folder or the image... thing (the bottom two, not the first burn to disc option). I've tried erasing the disc (though it's new) just in case, but it tells me it cannot be erased.