What are "popcorn muscles"? ?

Denver Broncos WR Brandon Marshall and Miami LB Joey Porter have been having a war of words recently, going back and forth talking trash about the other. Marshall, in his most recent comments on Porter, said all of Porter's muscles are "popcorn muscles". The overall message of Marshall's response was the Porter... show more Denver Broncos WR Brandon Marshall and Miami LB Joey Porter have been having a war of words recently, going back and forth talking trash about the other. Marshall, in his most recent comments on Porter, said all of Porter's muscles are "popcorn muscles".

The overall message of Marshall's response was the Porter was soft, but the term popcorn muscles confuses me as I have never heard it before. What exactly are "popcorn muscles"? Does anyone know or did Marshall just make that up?
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