Php 不同质量的图像压缩问题

我正在制作压缩图像大小的代码，如下所示

<?php
function compress_image($source_url, $destination_url, $quality)
{
    $info = getimagesize($source_url);
    if ($info['mime'] == 'image/jpeg') {
        $image = imagecreatefromjpeg($source_url);
        imagejpeg($image, $destination_url, $quality);
    } elseif ($info['mime'] == 'image/gif') {
        $image = imagecreatefromgif($source_url);
        imagegif($image, $destination_url, $quality);
    } elseif ($info['mime'] == 'image/png') {
        $image = imagecreatefrompng($source_url);
        imagepng($image, $destination_url, $quality);
    }
    return $destination_url;
}
if ($_POST) {
    if ($_FILES["file"]["error"] > 0) {
        $error = $_FILES["file"]["error"];
    } else if (($_FILES["file"]["type"] == "image/gif") || ($_FILES["file"]["type"] == "image/jpeg") || ($_FILES["file"]["type"] == "image/png") || ($_FILES["file"]["type"] == "image/pjpeg")) {
        $url       = $_FILES["file"]["name"];
        $temp_file = $_FILES["file"]["tmp_name"];
        $filename  = compress_image($temp_file, $url, 80);
        rename($filename, 'image/' . $filename);
        $location = "image/" . $url;
        if (file_exists($location)) {
            header('Content-Description: File Transfer');
            header('Content-Type: application/octet-stream');
            header('Content-Disposition: attachment; filename=' . basename($location));
            header('Expires: 0');
            header('Cache-Control: must-revalidate');
            header('Pragma: public');
            header('Content-Length: ' . filesize($location));
            ob_clean();
            flush();
            readfile($location);
            exit;
        }
    } else {
        $error = "Uploaded image should be jpg or gif or png";
    }
}
?>

压缩图像大小很好，但当我将图像质量（如80）更改为50、30、20时，它会显示相同的结果，例如，如果我为大小为“50、69、650字节”的图像设置图像质量80，它会将图像大小压缩为“40、34、652字节”，但当我设置质量10时，它会给我相同的图像大小“40、34、652字节”

我不知道为什么会这样

有人能帮我吗