imagepng 保存后图片非常达到1m多 原来仅是100k,该怎么处理 下面是代码

<?php
imgshuiyin("./img/1.jpg",'./img/6.jpg',100,100);

   function imgshuiyin($dname,$wname,$pos=7,$tou=50){
        //img(主图1，主图2，位置，透明度)
        function imgclass($name){
            $img_info=getimagesize($name);
            //getimagesize($dname)函数可以动态的获取图片类型，大小，宽度和高度等

            $img_width=$img_info[0];
            $img_height=$img_info[1];
            $img_mime=$img_info['mime'];
            //根据类型来判断资源
            switch ($img_mime) {
                case 'image/jpeg':
                    $img=imagecreatefromjpeg($name);
                    break;
                case 'image/gif':
                    $img=imagecreatefromgif($name);
                    break;
                case 'image/png':
                    $img=imagecreatefrompng($name);
                    break;
                case 'image/wbmp':
                    $img=imagecreatefromwbmp($name);
                    break;
            }
            return array('width'=>$img_width,'height'=>$img_height,'res'=>$img);
        }
        $dst_info=imgclass($dname);
        $water_info=imgclass($wname);
        //getimagesize($dname)函数可以动态的获取图片类型，大小，宽度和高度等

        $dst_width=$dst_info['width'];
        $dst_height=$dst_info['height'];

        $water_width=$water_info['width'];
        $water_height=$water_info['height'];
        $water=$water_info['res'];
        $dst=$dst_info['res'];

        /*
        输出：array(3) { ["width"]=> int(3104) ["height"]=> int(4192) ["mime"]=> string(10) "image/jpeg" }
              array(3) { ["width"]=> int(1470) ["height"]=> int(1391) ["mime"]=> string(10) "image/jpeg" }
         */
        switch ($pos) {
            case 1:
                $x=0;
                $y=0;
                break;

            case 2:
                $x=$dst_width/2-$water_width/2;
                $y=0;
                break;

            case 3:
                $x=$dst_width-$water_width;
                $y=0;
                break;

            case 4:
                $x=0;
                $y=$dst_height/2-$water_height/2;
                break;

            case 5:
                $x=$dst_width/2-$water_width/2;
                $y=$dst_height/2-$water_height/2;
                break;
            case 6:
                $x=$dst_width-$water_height;
                $y=$dst_height/2-$water_height/2;
                break;
            case 7:
                $x=0;
                $y=$dst_height-$water_height;
                break;
            case 8:
                $x=$dst_width/2-$water_height/2;
                $y=$dst_height-$water_height;
                break;
            case 9:
                $x=$dst_width-$water_height;
                $y=$dst_height-$water_height;
                break;

        }
        imagecopymerge($dst,$water,$x,$y,0,0,$water_width,$water_height,$tou);

        header("content-type:image/png");

        imagepng($dst,'./img/6.jpg');//可以自定义参数imagepng($dst,1.png);会自动生成1.png图像,最好随机生成
        imagedestroy($dst);
        imagedestroy($water);
    }