this depends on your type of harness.
If you use a collar and hames harnes, the goat will pull the weight with it's shoulders. If you use a breastplate harness the goat the pressure of the pull will be transferred to the sternum and/or windpipe.
It also depends on the type of load. A heavy load has to pulled with a collar and hames harness.
And it depends on the conformation of the goat: many goats are too short in the region where the windpipe joins the chest to fit a breastplate properly.
NEVER! is a goat pulling with the girth. And what do you mean with "where it is tightest"?