What does Mitt Romney’s “yams” mean?

There was the following passage in Vanity Fair's (May 16) article titled, "Mitt “Bird Legs” Romney is ready for his boxing match.”:

Romney also revealed two nicknames. As a high-schooler, he was referred to as “Bird Legs,” a moniker Romney imagines will make a reappearance after viewers of the fight see his yams. One of his sons has bestowed him with a new nickname: “The Glove.” As Leibovich quickly catches on, it’s a play on the name “Mitt.”

http://www.vanityfair.com/news/2015/05/mitt-romney-boxing-match?mbid=

I’m not able to relate the meaning of “yam” other than to “the edible starchy tuber of a climbing plant widely distributed in tropical or subtropical countries, a sweet potato,” as defined in COD.

What does “yams” in the above quote mean? Is it a slang for biceps muscles, or just a typo of “arms”?

It is almost certainly a typo, for gams.

Based on the context of the paragraph, I would say without a doubt that "yams" is slang for "legs," although it is not a term I have heard used in that way in my part of the United States.

One does occasionally still hear the very similar term "gams" to refer to legs, but almost exclusively in reference to a woman's legs, not a man's, and mostly in a playful or ironic "old timey" way.

Edit: a quick Google of "yams legs slang" does turn up some uses of "yams" as slang for legs, so the usage above is probably not a typo.