Changing png compression (level) in gdImagePngPtrEx doesn't affect the result...

Hi

I am using LibGD from the lisp environment (cl-gd) and I have problems
compressing png files when I create a new image by copying a part of
original png file.
What happens under the lisp wrapper is a sequence of gdImageCopyResampled
and gdImagePngPtrEx.
If I vary the level variable from 0 to 9 (which is called "compression" in
lisp wrapper) the size of resulting png image stays the same.
I am wondering if I can do anything to compress the final image? Does it
happen because the original png image already compressed and gdImagePngPtrEx
can't do any further compression? The size I am getting is around 95K. When
I try to trim it using optipng it sheds another 18%. If I convert it to jpg
(with 100% quality) the image is around 25K.
I am wondering can I compress it to say 50K using gdImagePngPtrEx? I am just
trying to optimize it for web and 95K looks a bit bigger than I want for a
400x400 image.
The reason I don't want to use jpeg is that when I convert it to jpeg, the
red color becomes blotchy even with 100% quality.
Any suggestions are welcome!

Thank you,
Andrew