They are called 501 S.T.Fs for a reason. As in Shrink To Fit. Usually on the jeans they have a label for you to sit in a warm bath with the jeans on so they shrink and conform to your body.
So in essence, you purchase them large, then shrink them to fit around your body. As in Shrink-To-Fit.
YOU'RE A FUCKING IDIOT.