Below a certain size, propellers are less efficient. It's a fluid dynamics thing. I learned about this from a colleague flying robots inspired by insects. If yo

Below a certain size, propellers are less efficient. It's a fluid dynamics thing. I learned about this from a colleague flying robots inspired by insects. If you find an article about bee flight dynamics that's not BS (creationists still sometimes try to use bumblebees as an example of divine magic), it may also explain it. The short version is that with a small propeller all the air around it just starts spinning.