I use Movie Edit Pro and I don't add any sound but the audio seems to take up 1/3 of the size of the movie. Is there a way to save the movie without any audio and not add all that size for audio that doesn't exist?
It really depends on which version of Movie edit pro you have. If it is one before MEP 2013 and 2014, select the video which includes the audio track, then "ungroup" the video from the audio and delete the audio track, or just "mute it if you think you might need it again. For some reason in MEP 2013 and 2014 Magix dedided to put the audio and video on one track. There is a way to do it, but I don't use 2013 or 2014 so I will contact a moderator and see if they can help.
Assuming you are exporting the movie - in the export dialog there is the option to export Video and Audio, Audio only or Video only as shown below - however there is a small gotcha - some export formats require the audio channel even if there is no sound and the option is greyed out.