DVD capacity problem


I know my way around computers but I’m just starting out on recording DVDs. I’m using Pinnacle Studio Deluxe 8 (software and bundled capture card) in conjunction with a Sony DRU-500A DVD recorder. I’ve rendered a video clip from an original VHS source which shows a file size (.mpg) of 4.07 GHz. I’m attempting to record this to a DVD-R disk which I understand has a capacity of 4.7 GHZ. However, my software tells me my clip won’t fit on the blank disk. As a check, I tried using Roxio’s DVD Builder and I get the same message. By my reckoning I ought to have at least 600 MB spare capacity. Is additional space on the disk required for encoding the clip in addition to the basic MPG file? If so, in general how do I calculate the total capacity required for any recording session. Pinnacle’s software has an option to make the clip fit on the disk but it renders the video at a lower quality. When I do that it shows 88 mins used and 37 mins free. It’s all something of a mystery to me. Can anyone explain please? TIA.