My experience is it all comes down to availability. I have tried other dishes when I did not like what I ordered and had the Bistro sandwich as a afternoon snack in First Class.

Also, I have asked for a second plate in Clubworld and gotten it. If they have the extra food, you can ask for seconds.

On BA they are very generous with portions and passengers regardless of cabin. I could not say the same for AA, Delta or United though.