I don't think it's just about size. There are probably GGG pipes that are as big or bigger than some great lines. If the pipes you're looking at are listed online, you might be able to compare the sizes. Great lines are generally, but now always large. They also tend to fall within a narrow range of shapes, e.g. mostly dublins and sometimes bulldogs. They often have plateau rims. Great lines are considered a higher grade by Castello. If they label one pipe a GGG and another pipe of comparable size a great line, they consider the GGG a lesser grade pipe. Whether there's really a difference, I don't know. They will certainly smoke the same.