Since a few months, RealFaviconGenerator can compress your favicon pictures for reduced bandwidth consumption. However, this option lacked three features:
This dark age is over with the new compression preview. The favicon editor now presents your master picture compressed with different quality settings.
If the differences between the original picture and the compress one are hard to spot, you can easily compare them with the zoom.
For now, the "expected compression rate" is not that accurate. I expect to improve it in a few month when enough favicons will have been compressed to establish reliable stats.