I’m having a similar problem. I’m uploading an optimize 8 bit png and all the generated sizes are 24 bit. For example, an original 720×720 png was 19K, however, the resized 220×220 was 29K, 330×330 51K, and 460×460 111K. Not only are the thumbnails significantly larger than the original but they are not optimized with lossless compression. I ran them through a compression utility which cut the size by about 40%! I’ve searched the internet for hours but have only found one reference to the problem and no solution.
