Overweight person here, but not from McD.
When I go to mac donalds, I get a hamburger and a diet soda (I don't really care for the fries).
Makes sense for me, a 500-600 calorie meal. I't a nice lunch, tastes good (all beef, even MCD, is awesome this side of the world), and even has lettuce and tomato.
In your example, that double big mac has 700 calories. Not a diet meal, but not that excessive. It even has a lot of lettuce, which is good against blood sugar spikes, esp. a good thing for most fat people. A diet coke is zero cal,, but a large coke in the US add 300 calories, reaching 1000 which is too much for a single meal. You are right that large fries, at 500, are not a good idea, though.