Ähm, das könnte evtl. daran liegen, dass MTV keine Kompression besitzt.
"MTV Video Format
The MTV Video format was created to allow video playback capabilities on inexpensive mp3 players without increasing the cost of those players. The format operates by displaying a series of raw image frames while playing MP3 audio.
The MTV format consists of a 512 byte file header, followed by alternating image and audio frames. While the audio frames are encoded with MPEG1 Audio Layer 3 (aka MP3), the image information is stored in uncompressed raw format, according to the player's display hardware specification.
During decoding of the video stream, the audio frames are passed to the mp3 hardware decoder, while the memory pointer of the display hardware is simply adjusted to the next raw image within the video stream. While this concept does not require additional * hardware for the decoding process, it leads to huge memory requirements as no compression is applied to the image information. For this reason the storage capacity of an MP4 player is effectively much less than that of a player that decompresses files on the fly."
http://en.wikipedia.org/wiki/ACT_(audio_format)#MTV_Video_Format