Catwalks are for sales to buyers in the industry. So are the designers' catalogs. Clothes shown on a thin models are more likely to be chosen by those buyers than ones displayed on a more average sized model.
I can tell you if all models are required to be size 8 or larger, only very very tall size 8 models will be used because they will have a visual advantage that appeals to buyers. Will girls be lining up for leg bone extensions?
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