It isn't purely weight that is the problem. I have no trouble playing the whole of the Saddleworth Rushcart route which is walking (occasionaly running!) at a fair speed whilst playing mostly bog standard tunes in G. It's when you start having to stretch for accidentals that you start to notice the effort involved.
A heavier instrument means more inertia to overcome when reversing direction so more effort involved particularly when playing fast tunes, and without checking the mathematics I suspect that this is not a linear scale effect but a square or even a cube effect.
Also the button top profile can cause problems. My 1962 Wheatstone has buttons that are more domed/less flat and this make them uncomfortable to play after about 5/6 hours.