It's easy to imagine that a person's forearm is like that of an artist's mannequin: symmetrical and wide below the elbow, and narrowest right at the joint.
It's not, though.
Certain muscles on the lateral side of the joint (the brachioradialis and the extensor carpi radialis longus) begin above the elbow joint on the humerus, creating a bulge which is widest at the elbow, not above it.
Once you see this, you'll draw forearms better.
And I think this arrangement of muscles gives a perception of the forearm bones being longer than they actually are.