what is the format of the video now? Is it xvid, divx or wmv?
if so then when trying to put them onto disc as a vob, they have to be rendered (authored) to mpeg2.
This will make the file size much larger.
also, Audio format is big in file size. If it is pcm audio it will take alot of space, you can shrink the audio size by making it dolby digital 2 channel.
I think it is just the conversion from whatever format you are in now, and I suspect it is wmv or divx to mpeg2. There will be a large difference.
Oh and have fun waiting for that render, 2 meg of ? to 7 mb of mpeg 2, it is going to take awhile.