Well, taking other welfare aspects into consideration, apart from staying sound with carrying weight.....you really shouldn't canter your horse, assuming it is you and your horse, all his weight PLUS yours is on one leg only, repeatedly. I'd say trot, or even better, walk only............
Sounds stupid and unreasonable? Well, but it's taking animal welfare first.
As for Golden's mare, I would really like to know the circumference of her cannonbone, and calculate with the method I posted. Which, by the way, DOESNT favour a heavier horse. Quite the opposite.
And another.....icelandics...as for not extreme work.......they do pace and toelt race speed, with adult riders, and the really fast ones are really fineboned and light.
So, to sum it all up........it all depends........
On type of horse, ability of rider, breed of horse, amount of work, type of work, condition of horse.
And for the welfare aspect, I don't think anybody here on this forum would knowingly and ignorantly overwork or overload a horse, welfare laws in place or not.