it's just a guide, and the sizing for the most part only varies with the heavier models. i also find BiG's sizing guide to be pretty inaccurate, considering they haven't updated any of them. it's probably a good idea to confirm sizing from whom ever you purchase from.
But by rule of thumb, excluding all external information, you would want to go true to size on the models. Also, BiG updates the measurement tables quite often, and Gordon is very helpful. If you are ordering from japan or take5, you always want to ask them to make sure that the waist measures XX (what you want)on the pair that they ship to you. I say that because samurai sizing differs from pair to pair rather significantly, more than other brands.