Here's why:
* Metaphor: A metaphor directly compares two unlike things without using "like" or "as." For example, "He is a lion in battle" compares the man's bravery to a lion's ferocity.
* "His brawny arms" simply describes the physical characteristic of the arms. It doesn't compare them to anything else.
This phrase could be part of a sentence that contains a metaphor, but on its own, it is a literal description.