A PNG is a compressed and lossless image format.
Here information about PNG compression can be found: http://optipng.sourceforge.net/pngtech/optipng.html
See also this question: https://superuser.com/questions/845394/how-is-png-lossless-given-that-it-has-a-compression-parameter
How can I set the compression degree (1 to 9) when exporting an image to PNG format?
I saved a bitmap Image using XnView with different compression degrees to png format. It seems that Mathematica is using as most of the software a default value of 6.