This appears to be a duplicate of PNG Compression and ImageResizer.
To sum up - PNG encoding is absolutely nothing like Jpeg encoding.
Fully optimizing the encoding of a PNG file takes lots of time and memory. You can't resize an image without decoding and re-encoding it. ImageResizer prefers to get the image back to you fast (~100-250ms) with a larger file size, instead of taking 5-15 seconds and using all of your server's RAM to generate a smaller PNG. Consider how long tinypng.org takes to optimize your image. Would you be OK with that kind of delay, in real time, on your website?
ImageResizer offers WebP, which can give you a better speed/size balance than PNG.
Basically, you won't get much file size benefit out of using ImageResizer on pre-optimized PNG sprites.