I take tons of screenshots on my Mac using the key combination
cmd-shift-4 and after I switched from my old Apple Cinema Display which wasn’t retina to the new Studio Display which is 5K I noticed the size of some screenshots was HUGE.
macOS by default uses PNG as the format for screenshots, and depending on what it’s in the screenshot, you can have like 10MB screenshots very easily.
Run this command in the terminal:
defaults write com.apple.screencapture type jpg
This will make the screenshots use JPG for the format, which will be much more lightweight.