From my observation, it's not so much that American portions are larger, but that American restaurants offer many extra courses. In America, you get bread or rolls while you are waiting to order your meal, plus maybe some coleslaw, olives, etc. You then get a soup and a salad. After, you get your entree, plus a couple of different vegetables on the side.
In England, you sit down, have your entree, pay for your meal, and leave. The entree isn't any smaller than an American entree, you just don't get rolls, soup, salad, etc. (At least that's how I've observed it.)