When you capture a screenshot on your Mac, it will be saved in the PNG-32 format, with support for 16 million distinct colors and transparency. This means that the screenshot will perfectly capture every pixel on your screen, but having four 8-bit channels for red, green, blue and alpha (transparency) for every pixel makes the file very large. If you're interested, you can verify this yourself using pngcheck.

The first step is to reduce the color palette of the screenshot. This is a type of lossy compression called color quantization, which will reduce the number of distinct colors in the image. The pngquant command line utility is the perfect tool for this job

In 2013, Google released zopfli, which claimed to improve compression by 3-8% compared to zlib. The trade off for this improvement: waiting an extra 1-2 seconds. (There is no decompression penalty when viewing the compressed image).

pngquant 64 --skip-if-larger --strip --ext=.png --force "$1" zopflipng -y "$1" "$1"