Оно "сжимает" с этими параметрами:  https://github.com/nothings/stb/blob/master/stb_image_write....

Исходный был с compression_level = 8? RLE compression ?  png_filter = -1;  

...
 where the callback is: void stbi_write_func(void *context, void *data, int size);
   You can configure it with these global variables:
      int stbi_write_tga_with_rle;             // defaults to true; set to 0 to disable RLE
      int stbi_write_png_compression_level;    // defaults to 8; set to higher for more compression
      int stbi_write_force_png_filter;         // defaults to -1; set to 0..5 to force a filter mode
...