The saying goes: "form fits function". This is one of the reasons why the human femur is larger and thicker than the humerus. If humans were to walk on "all fours" (like a dog or a cat), how might we expect our bone sizes to evolve

Our superior extremities would have grown more larger but not thicker than ourselves.

Explanation: We haven't to use our superior extremities to move. But if humans were to walk on "all fours", we expect our bone sizes to evolve as form fits function

a unit of heredity that is transferred from a parent to offspring and is held to determine some characteristic of the offspring.