ffmpeg -i input.vob -c:v copy -c:a copy output.mpg
This command does not reduce the size or alter quality. This merely demuxes the VOB and repackages it as an MPEG. You should be left with exact same quality. The size changes minimally due to losing the overhead of the VOB container.
If you want to maintain quality and reduce size you are going to have to convert it with another encoder like x264 or XviD to an MKV or MP4 container, for example:
ffmpeg -i input.vob -c:v libx264 -c:a aac -strict experimental output.mp4
This will give you H.264 video and AAC audio. Set
-crf 23 for the video quality, where less means better quality (sane values are from 18–28). The audio quality can be changed with
-b:a 192k for fixed bitrate, or if you want to use another encoder such as
-c:a libfaac, you can choose VBR with
-q:a 100 (100% default quality).