Large, of gigantic size. Certain large bones of antediluvian animals were at one time said to be the bones of the heroes who fell with Roland in Roncesvalles. “Rounceval peas” are those large peas called “marrowfats,” and a very large woman is called a rouncival.
“Hereof, I take it, it comes that seeing a great woman, we say she is a rouncival.” - Mandeville.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894