I'm using a simple thumbnailing script I wrote and it's pretty standard:

$imgbuffer = imagecreatetruecolor($thumbwidth, $thumbheight);
switch($type) {
  case 1: $image = imagecreatefromgif($img); break;
  case 2: $image = imagecreatefromjpeg($img); break;
  case 3: $image = imagecreatefrompng($img); break;
  case 6: $image = imagecreatefrombmp($img); break;
  case 15: $image = imagecreatefromwbmp($img); break;
  default: return log_error("Tried to create thumbnail from $img: not a valid image");
}
imagecopyresampled($imgbuffer, $image, 0, 0, 0, 0, $thumbwidth, $thumbheight, $width, $height);
$output = imagepng($imgbuffer, "$album/thumbs/$imgname.png", 9);

9 is the lowest quality setting, yet from a 400 x 600 JPEG image (at 56kB) I'm getting a thumbnail 27 kB in size (140 x 140). Using imagejpeg (quality of 80) instead of imagepng it's about 4kB.

How can this be, especially at the lowest quality setting for imagepng? I tried using imagecopy instead of imagecopyresampled, and imagecreate instead of the true color version. Unfortunately the images come out mangled somehow.

Is there any way to get PNG thumbnails of a reasonably small file size (about 4 kB at 140 x 140)? Or do I have to use JPEG?