theory on yarn shortages

so, of the people who have come up short on yarn, or needed more than the pattern called for, what size were you making? i made the S and needed about one-half to three-quarters of a skein more than what the pattern called for and in looking at Mari's post, she needed 1 and a half more, so i am wondering if the further you get from the model-size, the further off the yarn requirements are?