DVD's are like hard disk drives as far as quoted space is concerned. A 4.7 GB disk actually only holds 4.36 GB.

I'm guessing that your program is seeing how much space is available on the disk & tries to maximise the quality versus the space available. If you want to fit more on then why not add more videos at the same time rather than do it in sessions. I presume then if you're burning sessions then you're using DVD+ format.
The more you compress then the less space it will take up. TMPG is supposed to be quite a reasonable program & is shareware. It allows you a certain no. of burns before you need to buy it. Check it out to see what you think. Else look for alternatives here: http://www.videohelp.com/ or http://www.videohelp.com/tools