Nero adds to file size

[Flying Z]

I have just upgraded to the latest Nero Burning Rom suite, (had this problem before though) and I record VHS tapes I own to backup in MPEG 2 format and have set the recording options to be the same (aprox) as nero uses when it transcodes. My question is, how come my video file is say 2.25 gb but Nero makes it about 3.5 gb? I use the setting Extended play which is supposed to give 240 minutes of play but i only can get about 150. Any suggestions? Thanks