This would be easier if girls didn't get so defensive about their weight. Another problem is weight doesn't tell the whole story. See, a toned 130lb chick could look smaller than chubby 115 or 120lb girl just because muscles are twice as sense as fat.
A general "rule" is about 20%-25% body fat. More fat than that will look unhealthy and make a girl look bigger than a muscular girl who weighs more.
Sorry this doesn't answer your question, but without a height to judge the weight, the question isn't really that informative.