Jan O

There are "lossy" ways of compressing PNGs. Much like you can compress a jpeg more by throwing away information, there are tools that can throw away some png information to make the image look similar, but be much smaller. If I'm ok with losing information (eg. reducing the colour palette in a lossy manner) then you can try a lossy tool like pngquant.

That depends on how inefficient the original compressor was. Rafał M's experience of 1-15% sounds about right, but if the png is the output of another compressor (eg. pngquant) then expect less than 1%, if any.