The different sizes come in different widths. This is pretty common practice in the ski industry. If you do not scale all dimensions on a ski with the length, you end up with dimensions that don't work, or sidecut that is just off. I know the 191 size is a 105 waist, so the 104 waist must be on a shorter length.
Some companies just keep standard dimensions across all sizes, whereas others, like the Head supermojo apparently, scale everything for each size, so its all good. Here, check these out. Notice the different sidecut and dimensions for the two sizes.
another good example is the Line EP mini. If it kept the tip waist tail dimensions of its big brother, it would look stupid, and perform just as badly. So they scale the width, tip and tail to match the length change.
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