Sample size clothing is generally made in a size eight, and they ask for models a size smaller to make sure the models will fit - it's easier to pin things smaller than work around too-small. So I'd bet they've got samples donated that won't fit someone smaller.

Hmmm, maybe. But if their "artistic and physical constraints" are "we already have the costumes, and you'll have to be able to fit them", why not just say so in plain language?

ALso, note that this is only the casting for four of the clans

Right, because apparently they have already cast the other nine. I'm curious what sexes they are and what the requirements were.

and they aren't even asking for males

My guess is they divvied up the clans and had enough men already on staff or in easy reach, and females were a little harder to come by.

it's possible that they did only use males with 28" waists and that some of the other models are larger.

It's possible. I hope they did lay a restriction on the men that equates to "we only want size 6 women", or that the case is "we have several larger women and now we need several smaller ones". Still, the way it's worded, I have a hard time not getting pretty pissed off. If it's a costume thing, maybe they should have waited until after they had the people that had the perfect "I think we've found our Gigi Malkie" faces, eh?