[asddkk]

I live and breathe IRT models of the sort they discuss for my work and it's fascinating to me to see it applied to clothing sizing. It makes sense because it's a measurement problem.

One thing they don't get into really is that the IRT model they are using is pretty simple, which is typical of the random/mixed effects model formulation as a way of keeping things mathematically tractable to permit parameter explanation/prediction.

They could add other components to the model, though, such as different dimensions of fit (different aspects of body shape and fit), or how closely different items track those dimensions (as opposed to just how large or how small; e.g., maybe some types of fabric provide more information about fit than others). The models they're fitting are a sort of entry point in that regard.

It wouldn't solve the problem of course, and I agree with you 100% about in-person fitting being the final word, but they also have a lot of ways they could improve these models.