There are many variables that come into play in an armwrestling match, but one answer as to why a person with a larger hand and wrist has an advantage in armwrestling has less to do with strength, and more to do with leverage. In armwrestling, you and your opponent lock hands and attempt to pin each other to a pad on either side of the table. To pin your opponent, you must pull his arm/hand down. Doing so requires not only enough strength to move his arm, but also a secure grip of his hand and wrist. The simple fact that a thick hand and wrist are more difficult to maintain a secure grip on give a person with a thick hand an advantage, and the fact that a long hand can cover more of an opponent's hand gives the person with a longer hand an advantage. Imagine palming a basketball vs palming a shot put. Why is it more difficult to maintain control of a 22 ounce basketball with one hand than it is to maintain control of a 16 pound shot put? You can get a more secure grip on the shotput, that's why.
In the elevator example, you are correct. The elevator floor does work on you as you stand in a rising elevator.
In the jumping example, the floor is not doing work. After all, it is just a floor, it has nowhere to get the energy from to do work on you. (The floor of the elevator gets it from the motor running the elevator.) When you jump, the force from the floor on your shoes increases, but there is no displacement between the floor and your shoes until your shoes leave the floor, and at that point there's no force any more. Your torso is moving upwards while you jump, but there's no force from the floor on your torso, so the floor isn't doing work. All the energy of a jump comes from internal energy, not external work.
In fact, the floor does a bit of negative work when you jump, taking energy away from you, because it deflects a bit, moving down as you jump up.
When you lift an object off a table, the table does a tiny bit of work on the object for a similar reason. When the object sits on the table, the table deforms a bit. As you lift, the table goes back to its original shape, so there's a small bit of force*distance work being done. The deflection is usually so small as to be unnoticeable. On a trampoline it is much larger; enough to make jumping a very different experience.
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FGSUZ and Martin Ueding answers already cover the basics of torque. But a rough diagram might be helpful. Answering directly the question
It's due to reduced lever arm $L$, so yes, it's due to the angle between the forearm and the elbow, but only to the extent to which this angle leads to the reduction of $L$ from $L_\mathrm{extended}$ to $L_\mathrm{bent}$.