Egg McMuffin and coffee for breakfast and then a Big Mac, medium fries and iced tea (or diet Coke or water) for both lunch and dinner is... 2130 calories. Not the pinnacle of the healthiest diet for sure. Over 30 days for a mid-sized dude, you might gain... one pound.
Absolutely, drinking diet coke or water is cutting out an enormous number of calories, as-is medium fries.
Heck, I only weight 185lbs and back when I ate mcDonald's a single Egg McMuffin wasn't nearly enough food to make me feel full. I'd get two and a hash brown, and still be hungry and hour later.
In the sister comment, I did some math and because the guy was 188cm and 220 pounds to start with, he needs 2400 calories per day just to keep his weight. He would have lost at least half a pound per week with the diet you just mentioned.
