VOB different size than combined .mpv & .mpa?

Why is it that the .VOB that is created by my authoring sotware is larger than the total sizes of the .mpa and .mpv?

stream data (extra info) is added during muxing

what is included in the stream data, or extra info?

The primary reason I’m curious is because I’m wondering if there’s a way I can calculate what the total size of the .VOB will be BEFORE i begin encoding the ORIGINAL content into MPEG-2