Quoting (again) here:
"Chienworks: The output file size is determined by two factors: duration and bitrate. If you want a smaller file then make it shorter or use a lower bitrate, or both."
Unfortunately Make Movie > Burn it to a DVD does not allow you to customize the bitrate. You need to go to Make Movie > Save it to my hard drive > Render as, or (in versions up to MSP 12) to Project > Render as. These options allow you to customize the templates.
A "Search" on the forums would give you links such as those mentioned here.
Some sections here might help to understand as well.
Unfortunately that document is not on this site any more.