Yep, JonBenet would have willingly worn panties so big they would have bunched up and caused bulges to be seen under her black velvet pants. NOT! JonBenet normally wore sizes four and six. She was tiny. There's no way she wore size 12 panties to the White's dinner party.
Also, JonBenet didn't put the panties on in the morning that she wore to the Whites that evening. Patsy says JonBenet didn't get a chance to get a bath, but she did change underwear prior to going to the Whites.
Those size 12 panties were obviously put on JonBenet by the person who had just killed her. It was part of the staging to naively try to hide the sexual assault.