起因

Mac自带截屏功能输出的图片尺寸过大，原因是其截屏功能默认分辨率为视网膜屏幕的，并且截图格式为PNG。所以为了压缩图片大小，考虑让Mac截屏的输出图片编程JPG格式。

The size of output images generated by the built-in ScreenShot of Mac are too large. The reason could be that the default resolution of these images is retina, and the format of the screen capture isPNG. In order to compress the image size, try to configure the output image format as JPG.

经过

使用Mac终端输入以下任意一个命令即可：
Run either of the following commands on the Mac terminal:

defaults write com.apple.screencapture type jpg

defaults write com.apple.screencapture type gif

defaults write com.apple.screencapture type png

defaults write com.apple.screencapture type pdf

结果

如果使用以下命令，输出成jpg图片：
By using the command below, we can get JPG screenshots:

defaults write com.apple.screencapture type jpg

图片的大小能够获得有效的控制。
Then we have control of oversize of screenshots.

PS: 苹果公司设计的软件真的好蠢。
Dumb Apple.