Why does it really matter if proof is given or not to support the statement that it is a proven fact that if customers stop ordering large portion sizes, then the restaurant will stop offering them?
Or, what does it matter whether the customers are following the restaurant's menu choice changes or the restaurant is following the customers' ordering patterns? (The chicken or the egg?)
Well, to me at least, I have very little stake (or steak) in the outcome of lack of outcome in any empirical study of customer dining habits. If I owned a restaurant I would certainly have more interest.
As to your second statement: As long as I can order what I want, or choose not to order at all, then I don’t care whether the restaurant is secretly trying to manipulate me. I know how to say “No, I don’t want you to jumbo-size my order.” Alternatively, maybe I want them to jumbo size it for an extra 50 cents or whatever, and save half for later. It’s win-win.
To paraphrase Bill Murray, “It just doesn’t matter!”