There was no charge for an extra mains when we sailed the Med last October (and that’s since the policy was introduced on US sailings). You could order as much as you liked from the menu.
The menus do differ slightly as MSC tries to accommodate American tastes on US sailings. Also, the US dollar is the currency on those cruises, whereas it’s Euros on European sailings (and, I believe, it can be Sterling on UK cruises).