Rendered File Size Doubles

I am using version 11. I rendered 6 files from project 1 a few days ago using Windows Media Video V11 with the 3 Mbps Video setting selected. A 57:14 minute clip was 621,549 KB. Two days later I output 7 files from a new project, created using the same settings as the first project, using the same WMV V11 template. A 53:16 minute clip is 1,047,993 KB. A 1:01:28 clip is 1,414,137KB. All of the video clips rendered from the second project are almost twice as large as the files rendered from the first project. Can anyone tell me why this may have happened? I have checked and rechecked the settings and the properties of the rendered files and the properties of the project files are identical - it is only the file size that varies.

In order to see if there was a problem with my copy of Vegas Pro ll, I rendered one of the files from project 1 again and it rendered with the same settings as before and it is the same size it was before - so it looks like it is not a problem with my copy/installation of Vegas Pro.

The only difference between the two projects - besides one being about 15 minutes longer than the other one - is that I pulled one of the audio files into Sound Forge to eliminate some background noise. Otherwise, both are identical, shot with the same camera, using the same settings. Both include tiff files created from PowerPoint slides that I have dropped into a video track throughout the video.

Any suggestions will be appreciated.