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:
1. Use the classic Windows theme
2. Do not use a wallpaper
3. Record at 800x600 or 1024x768 pixels resolution
4. Turn off windows animations like Fades and Scrolls