I would guess that you have the capture settings on default which is only 50% quality level. To change this, click on the capture video option the click on the configure button and then select the video setting option. Move the sliding scale to 1.000. This makes for the best quality capture but you will start with a bigger file size. BUT this will now match the best rendered file size when you finish the project.
Some people get smaller file sizes is by using a codec like mpeg2 that compresses the data in the file. With the right settings (practice) you can do this without to much picture quality loss. In WMM choosing a .wmv should give you a similar result. BUT please under stand that WMM is very limited in the settings you can use, so you may not be able to get the best results from it.