NY explained on 06/08/2019 :
I presume if the outlet pipe is kept vertical, there is less chance of air
bubbles getting in at the bottom and rising to the top of the loop, breaking
the siphon.

Correct, the more vertical the pipe, the less chance of the syphonic
action being disturbed, but its difficult to have a large long pipe
vertical for any appreciable distance.

I imagine few of us have ever used a very fat pipe for siphoning - normally
it is something about the diameter of a hosepipe.

Even an hosepipe size, suffers the syphon action being disturbed.


Is it purely the diameter that causes air to bubble back up, or is it the
ratio of the diameter to the length? In other words, is a short fat pipe more
likely to do it than a longer pipe of the same diameter?

The water's skin (name?) is the main thing which allows it to work well
in the smaller pipes. Add in some soap, disrupt the skin and then you
have more issues syphoning with even a small diameter pipe.