I will scale the icons proportionally in version 1.9.3 (to be released).
Unfortunately the icons for Videos, Music and Images are in low resolution only and they look a bit blurry when scaled.
If anyone wanted to disable this feature. They can set a hidden tweak via Terminal.app
defaults write com.binaryage.totalfinder TotalFinderNoSidebarIconsScaling -bool YES
You can alternatively specify your own icons via ~/.totalfinder-sidebar-icons.plist by following this post: