Hello.
I have a new suggestion. The internal plugin WideImage ist set to 100% quality for writing uploaded JPGs. This blows the filesize unnecessary up, even with a lower picture size and the quality isn't better.
I think 60-70% is ok and should be set in the next release.
/ow_libraries/wideimage/Mapper/JPEG.php
function save($handle, $uri = null, $quality = 100)
to
function save($handle, $uri = null, $quality = 60)
PNG is already set to maximum compression, but this is no problem as it is a lossless format.