Originally Posted by usagi_tetsu
It's not a question of file size, it's a question of copyright infringement.
Which boils down to an issue of fair use under copyright laws. He is not selling the videos to others, but YT is capable of selling ads during playback. Since they are pseudo owners of the video, they can get in trouble for copyright issues if they run ads on the screen during the movie, which is why things get pulled. AFAIK, Vimeo doesn't do ads the same way.
If it came from the production company that might be a different beast, but there are tons of videos that use background music and are allowed to do so under fair use as long as they don't choose to run ads with the video.