On a man of average height, a 16 inch muscular arm is very impressive—a 17 inch upper arm is so large that it may make the individual appear freakish if the rest of the body's muscular proportion is not in proportion—an 18 inch upper must be seen to be appreciated—and a 19 inch upper arm approaches the impossible insofar as size is concerned.
True—I once saw a man with upper arms that were over 20 inches in reasonable muscular condition; but he wasn't an average individual—he was just under a full nine feet tall and weighted over 250kgs!
Brickhead
Please post a picture of your upper arm with a tape measure, relaxed and cold.