The issue is portion size. To prove the original point, proof would have to be provided that customers first stopped buying the items because the portion size was too large.
If you confuse the other factors, perceived value, taste, presentation, then you cannot prove the customers stopped buying simply because of portion size.
so you think portion size is somehow different from other aspects of business?! It’s simple capitalism that’s been around forever and was eventually written down by Adam Smith, if your stuff isn’t selling you figure you start asking questions, hire surveying companies, go through the archives of articles about your business, find out what people are complaining about and change it. If the complaints are portion size then you shrink them. Now it might or might not have happened yet, but the basic rules of capitalism say it CAN and WILL, at some point (especially now that the anti-fat crowd is turning large portion size into a negative) large portion size will (and possibly already has) driven away enough customers that the restaurant will change it.
Your demand for “proof” is just silly pointless arguing for the sake of arguing, either you don’t believe in basic capitalism or you know this can and possibly has happened. Just that simple.