I was going to counter this with gymnasts, since size past a point becomes a problem that can't be overcome by the larger man (it is better to be smaller), and then I realized that the idea still holds, there are certain limitations of which we cannot overcome no matter how hard we try.

People do not go to the gym to increase their height.They are focused on things like increasing the size of their bicep,so the weak/strong analogy holds better.

I was not talking about biceps size, but about who is stronger in a fight. The bigger guy has an advantage. See the other child comment as it explains that it can work the other way around (bigger being a disadvantage)

Your comparison is completely silly as it relies on something which can't be changed by determinated effort, unlike human brain and mental skills/capabilities and our physical body.

The comparison of going to gym holds perfectly, as anybody can go there and bulk themselves way, way bigger, fitter and more muscular with effort than almost anybody else who doesn't go there. Same is definitely possible with human brain as numerous studies show that after exposure to a new skill, learning it does physical changes to our brain(neuroplasticity) and thus modifies it to work better/more optimally, making it more capable.