Do the people who say we don't need carbs, and would be healthier without them never eat vegetables?
And what food group would you say the calories in a McDonalds meal typically come from
Fat/carbs, there's not a lot in it to be honest. Just put one of them special bacon double cheeseburger they're doing now, large fries
and large full sugar coke in their nutrition calculator and got 1335 cals, 61 g fat and 147 g carbohydrate, mostly from sugar in the coke. But there's also 44 g of protein and just because the beef and bacon is McDonalds, it doesn't make it nutritionally deficient. Still good protein like from any other animal source.
As there's 9 cals/g for fat and 4 for carbs, the calories from these will be about the same and you could substantially reduce the sugar and hence carbs by having a lower or no sugar drink.
But that's obviously too many calories for a meal for most people and that's the point. People are eating more than they used to, moving less and drinking more calories.
But it would depend what else a person eating that meal ate whether it caused them to gain weight. I would find a large bacon double cheeseburger and large fries extremely filling and couldn't eat it in one go, so it would probably count as 2 meals nearly. If I wanted to eat something like that, I would have a decent sized breakfast and a late-ish lunch of the monster burger and probably not eat for the rest of the day and as long as I did a little more than sit on my arse all day, it wouldn't cause me to gain weight. But obviously others would want to eat again that day, with that for lunch.