If you are an apparel marketer, then you know all about the importance of extended sizes.
Visit your local retail store, and you're not likely to find a broad assortment of size 14 dresses or XXL coats. Go online, and you can find absolutely anything you want.
The apparel marketer uses Multichannel Forensics to understand the unique behavior of the extended size customer.
From a marketing standpoint, you don't necessarily waste resources pushing the extended size customer into your store ... in many ways, you are limiting the ability of this customer to be a multichannel customer due to your assortment and merchandising strategy.
Extended size customers can be fiercely loyal to a brand when they find something that is fashionable and comfortable. Extended size customers can also be "leading indicators" of problems with your merchandise assortment, if loyalty wanes.
Analysis of extended size customers is easy --- simply identify, by size or sku, the items that are considered extended size items. Track customers who buy these items different than all other customers.