Originally Posted by
MaximusDamon
It seems to be pilot's discretion to some extent, because once they said they would have to ask the pilot if I could board.
Just to clarify, they don't ask the pilot because it's at his discretion; they ask because the pilots are the ones who do the weight calculations to determine whether or not there is room for one more. There are set weight limits, and all the pilot does is let the agent know whether the limits would be exceeded if another passenger/bag is added.