Compressing PNG Files
PNG files generated in SpotLight are generated as master assets with lossless compression. When compression is required, we recommend using the JPG output option in SpotLight, but when PNG and lossless compression are required, the pngquant library can achieve this for you. It is a command line tool and library for compressing PNG images, it can be integrated into any batch processing of assets or there is a GUI version available from here.