Php调整图像大小会增加图像大小

我正在创建上传文件的thumbail。

如果图像宽度和高度大于200，那么我将它们重新调整为200px。

这是我用来做的代码：

if (file_exists($old_file)) {
        $path_parts = pathinfo($old_file);
        $extension = $path_parts['extension'];
        $filename_path = $filepath . $filename;
        $destination_path = $filename_path;
        if (strtolower($extension) == "jpg" || strtolower($extension) == "jpeg") {
            $uploadedfile = $old_file;
            $src = imagecreatefromjpeg($uploadedfile);
        } else if (strtolower($extension) == "png") {
            $uploadedfile = $old_file;
            $src = imagecreatefrompng($uploadedfile);
        } else {
            $uploadedfile = $old_file;
            $src = imagecreatefromgif($uploadedfile);
        }
        list($width, $height) = getimagesize($uploadedfile);
        $newwidth = $Size['width'];
        $newheight = $Size['height'];
        if ($width <= $newwidth && $height <= $newheight) {
            $newwidth = $width;
            $newheight = $height;
            $tmp = imagecreatetruecolor($width, $height);
        } else {
            if ($width > $height) {
                $newheight = ($height / $width) * $newwidth;
                $tmp = imagecreatetruecolor($newwidth, $newheight);
            } else {
                $newwidth = ($width / $height) * $newheight;
                $tmp = imagecreatetruecolor($newwidth, $newheight);
            }
        }
        if ((strtolower($extension) == "png") OR (strtolower($extension) == "gif")) {
            imagealphablending($tmp, false);
            imagesavealpha($tmp, true);
            $transparent = imagecolorallocatealpha($tmp, 255, 255, 255, 127);
            imagefilledrectangle($tmp, 0, 0, $newwidth, $newheight, $transparent);
        }
        imagecopyresampled($tmp, $src, 0, 0, 0, 0, $newwidth, $newheight, $width, $height);
        if (strtolower($extension) == "jpg" || strtolower($extension) == "jpeg") {
            imagejpeg($tmp, $destination_path, 100);
        } elseif (strtolower($extension) == "png") {
            imagepng($tmp, $destination_path, 5);
        } else {
            imagegif($tmp, $destination_path);
        }
        chmod($destination_path, 0777);
        imagedestroy($src);
        imagedestroy($tmp);
        ob_flush();
        flush();
        ob_end_flush();
        return true;
    } else {
        return false;
    }

它将大图像的大小重新调整为200px乘以200px，但图像大小在中增加（字节和kb等增加）。

我尝试上传8kb的png文件，新的缩略图文件大小是28kb？

尝试过谷歌搜索，但没有发现任何有用的

谢谢。