I had strange results when trying to get good png compression.
I used Imagick::COMPRESSION_ZIP
Here are a few file sizes for a few compression levels.
Note the best result at 0. Also I got thrown off for a long time trying to get similar results to gimp. Make sure your image depth is set to 8, it appears to be 16 bits per channel, at least for me.
You can check with identify -verbose filename.png
You can also strip a bit more file size with $image->stripImage();