Most of the (pro/famous) strong climbers have lean body. Obviously being light helps in climbing. But climber has to be strong too, so some muscles are required after all. Can extra muscles compensate extra weight? What formula does explain the compromise between big muscles and body weight? Let us see.
As a climber you fight with gravity. Because (in simplified form) climbing means lifting your body up. In pulling your body up you apply force against the gravity pull.
In simple words gravity is a force of attraction between you and Earth. This force is proportional to ∝(M*m/r2) where M is Earth mass, r is Earth radius and m is your mass. M and r are fixed, so you can play only with your own mass (weight).
Mass is directly proportional to the volume of your body. Say you became twice bigger, that is twice taller, twice wider and twice thicker (i.e. got some real curves). How did your volume change? Volume is product of all dimensions and grows as a cube, thus the answer is 2x2x2=8. That also means that you became 8 times heavier. The gravity pull is 8 times stronger now.
How many times stronger are you now (after you grow twice in all directions)?
An individual’s physical strength is determined by the cross-sectional area of muscle fibers recruited to generate force and the intensity of the recruitment.
The cross-sectional area of muscle fibers is 2-dimensional area and hence in our example changes as 2×2=4. Means you became 4 times stronger.
Yes, you are 4 times stronger, but gravity force is 8 times bigger, so at the end you will do (8/4=2) 2 times less pull-ups than before.
Of course, our model is over-simplified, but you got the idea. Try to put 80 lb on your harness and do single pull-up. Most of us can’t do it, i.e. we can’t even pull 1.5 of our weight. On other hand bugs can pull and lift 10-1000 times of their weight. Reason? Just because we are big and bugs are small.
Read some comparison facts here: Human vs ant: Animal athletes in action
Conclusion: be careful with gaining weight, try to stay lean.
But do not get too crazy about your own weight, staying healthy and happy is more important than anything else.