With uniform quality control, they can reduce the portions of your fast food over the years. So no matter which outlet you go, it'll probably be the same small size. (Ok, it may be larger in th western countries) So you can't go and complain that a certain outlet short change you unless you want to take it out at all the outlets.
Fast-food is getting less and less attractive to eat. I now avoid Macs and KFC. Stick to the nearby coffeeshop. (Not even those franchised air-con kopitiam) The food you get never looks like that in the adverts. What looks big becomes smaller!