That amount of compression should be achievable by optimising the code. Unfortunately, the only tool I have click here is not able to optimise large files so gives up whenever I try to optimise a whole directory at once. Other tools may work better. The file size savings are small.
If you find large graphics with sort by file size, Irfan View will probably be able to compress them enough to save sufficient space without noticeable loss of quality. It is less work that it seems since you can batch process JPG files to the same compression ratio.