Big American-size Meal Portions

Went out for dinner last night with some colleagues. Ordered a prime rib sirloin and thought it was advertised as a 10 oz. steak. Well much to my surprise the supposedly 10 oz sirloin was more like 30 oz. Nothing like eating 30 oz of medium rare beef at 21:30. Why are meal portions so darn large here in the US of A? I was very hungry last night and did devour my meal unlike a female colleague of mine who could only eat about 1/3 of the hunk of beef in front of her. I would have taken the other 2/3 off her plate but I did not want to embarass myself in front of the others: Nothing like sitting down for a nice chunk of beef for dinner.