wideimage php图像压缩不适用于PNG？

I'm trying to compress my png image after it resized via wideimage php. but even I have set the compression level still it saving without compression.

But for jpg it works,pls advice

$img->resizeDown($size['maxWidth'], $size['maxHeight'], 'inside')
->saveToFile($fullpath, 7, PNG_NO_FILTER);