Why is Presenter Video Express for Mac generating such giant (900 MB) files?

A faculty member recording captioned video lectures using Presenter on Mac is ending up with huge MP4s (900MB and up for a 20 minute lecture). Does anyone know why this is happening? Even without captions the MP4s are ending up at about 250MB. Here's what we've ruled out:

• Small original PowerPoints (<14KB)
• Publish Quality set to about 75%

When I do a similar test on PC I end up with a 15MB file with captions for about 7 minutes of video. Does anyone have any thoughts about other factors that might be affecting file size?

Thanks,

Melissa