file enlarges after loading to nero...

hi, my movie file is 300mb right? but when i loaded to nero no burn a vcd, the 300mb expands to like 930mb or something like that... why? i don't understand. AND ..AND i also havce one of those "sony cybershot digital cameras" that can record movies files, and when i burn those, which are like 15mb each, and then load then on nero, they STAY at 15mb each. so then i try getting technical with it lol. i check the sound files in my movies in my cpu and they are mp3 with the sound files and mpeg4 (divx) with the video. on the other hand, with the sony camera files, the movies are regular mpg video and the audio are mpeg2. so i try converting my 300mg movies files to mpg-video and mpeg2-audio, and nothing changes, can some one please help me on this? maybe i'm doing something REALLY wrong please help me i'm killing my slef here lol thnx for everytrhing and anything