There was a Global Cycling Network (GCN) video put out a year or two ago where a number of the ex-racer presenters, having learned that Tour de France winner Alberto Contador often trained by climbing out of the saddle for half an hour at a time, tried to emulate that training technique. World Champion Emma Pooley, at about 100 pounds and five feet nothing, had no trouble climbing for the full half hour. The others found it more challenging. One of the other presenters, lanky and a bit over six feet in height, suffered through the ride--I don't remember whether he gave up or not, but he wasn't happy.
I suspect that people who can climb out of the saddle for extended periods have learned to shift their center of gravity backward such that their hands are supporting very little of their upper body weight. A bit tricky if you're not used to it, but much less fatiguing.