Flash is a vector-based format and a flash file size cannot be estimated accurately. WMV contains a sequence of highly-compressed images, which can be estimated to reasonable accuracy. In general, WMV, as implemented in My Screen Recorder Pro, will provide a significantly smaller file size. To reduce file size for SWF, make your screen-captures with the following settings: Use the classic Windows theme Do not use a wallpaper Record at 800x600 or 1024x768 pixels resolution Turn off windows animations like Fades and Scrolls