Why is there a glut of extra-large clothing?

Edward Casabian writes:

I'm a mostly loyal reader of Marginal Revolution and have a question for you.

Why is it that the vast majority items on sale in a clothing store are almost always XL or XXL?  I was in Old Navy last weekend and wanted to pick up a few t-shirts, but virtually all of them were too big for me.  The same went for shorts and jackets.  I am average size, about 5'9"

I would think that there would be more small, medium and large size clothing as these items would cost less to produce and seem to have a higher demand as evidenced by the inordinate amount of large clothing that is always on sale at department stores.

I believe the same goes with footwear.  The most popular sizes (9-11) always seem to be out of stock.

I can't vouch for these stylized facts but I do have the same casual impression. 

One simple hypothesis is that the less common sizes have more unpredictable demands, relative to inventory, and so they are more likely to end up in surplus.  They're also more likely to be unavailable when you need them, though perhaps that latter state of affairs is less noticeable.

I also question whether you will find an equivalent overrepresentation of XL at Banana Republic (I guess no) and what that means about the clientele of Old Navy.  The company which owns both may be pursuing a market segmentation/price discrimination strategy and Mr. Casabian is expressing his preference for more search and lower prices instead of reading MR all the time. 

The most general question is which clothes sizes should be most likely to experience oversupply.  My guess is that occurs when branding is least important and the possible durable goods monopoly breaks down.  Maybe people buying XL are less interested in brands (or brands are less interested in them) and thus their market is more likely to be flooded.

These are just my guesses; maybe Kathleen Fasanella would know the answer.