默认情况下ios是用4个点表示一个实际的像素点，4：1的关系，所以图像在ios设备上运行时，看上去要比实际用像素画的比较大。
文档里是这么说的。稍后追加
The different resolutions only come into play if
you want to use high res mode on the retina displays (iPhone 4, iPod
Touch 4th Gen).
If you enable high res mode, then you can/should use the larger textures for your map.
If you don't, then the retina display will use the same resolution as
the earlier models. (It uses 4 pixels of the display to display 1
"normal" pixel, so it works out to the same layout as "old" devices)
See Also:
http://www.cocos2d-iphone.org/forum/topic/8511
http://www.cocos2d-iphone.org/forum/topic/9315