Changing your Audio Source in MSI AfterBurner from DirectSound to WASAPI reduces captured video size
WARNING - I will NOT be held responsible for any damages the software(s) and methods shown in this video may cause to yours or someone else's computer, hardware, software, and/or data, including loss of use of such devices and/or loss of data and/or loss of profits from the resulting inability to use such devices and data. Any information or program you choose to put into use from this video is done AT YOUR OWN RISK.
In your MSI AfterBurner Video Capture settings, under the Audio Capture Properties, the Audio Source is by default DirectSound Capture Device. Changing this setting to a WASAPI Capture Device will greatly reduce the overall file size of your RivaTuner captured videos.
To learn more about MSI AfterBurner/RivaTuner:
How to record (capture) PC gameplay footage (video) using a free alternative to Fraps
http://www.youtube.com/watch?v=EOXZdFh26tI
How to encode, convert and compress video free of charge and easily using open source freeware
http://www.youtube.com/watch?v=VSqHjHUIbz0