Does the final 3.8gb file play okay & complete from start to end, including audio? If the audio was not extracted fully (not rare) then a small final size but the movie also incomplete. What Audio bitrate did you select during encoding and what bitrate did you use when converting the audio to AC3 ? If these are different, then there is your answer. Check that the Max Average is at 7200, if its lower then it has maxed out on that bitrate and not able to fill a dvdr.
I have been using Max @ 9000, Min at 200 & Max Average at 7200 for almost 2 years now and have NEVER had a small sized encode unless the movie was below 90min in length.