Similar Projects Same Settings Very Different Results

I convert recorded lectures for online viewing. I always use the same settings (with some experimentation mentioned in parentheses below) but the file sizes of the exported videos are wildly different. My settings, most of the time are:

• Windows Media
• NTSC DV
• 1920 x 1080 (same as source -- reducing this seems to increase the export time and file size)
• Bitrate Encoding: VBR, 1 pass (going from 2 pass to 1 pass solved a problem I was having a couple years ago)
• VBR Quality: 100 (lowering doesn't make a significant difference in file size but definitely lowers the quality too much but sometimes I'll try something different out of desperation)

The duration changes but it doesn't correlate to the difference in file size. For example, the follow videos use the above settings:

• 27 minutes = 340 MB
• 23 minutes = 1.26 GB
• 17 minutes = 1.22 GB

The longest videos I ever do are approximately one hour. Sometimes videos of 45-55 minutes are well below 1 GB, sometimes well above up to several GB.

I arrived this morning to find a 33 minute video export with a file size of just below 10 GB. That broke a record.

If anyone can offer any insight I'd be grateful. Thanks in advance.