How do people get overweight?

I know obviously the basics of how it happens. But more exactly the numbers expect don't add up? if we burn 1800 or so calories per day just while doing nothing 75 calories just by sitting for an hour x24 for the day 1800. So this is taking nothing else in account that you can do in a day that may burn more like just basic walking.
Then for example my dinner and lunch and breakfast would never hit 1800 calories in that food. Even if i eat some chocolate or something it wouldn't go above that really. So to get overweight would i need to sit here and just eat chocolate all day?

Even McDonalds meals only end up been around 1000 calories, so assuming you have mcdonalds for dinner then have a basic sandwhich for lunch and then some toast for breakfest your probably gonna end up under 1800.

And i doubt people have 2 mcdonalds meals everyday if they did then that would explain been overweight but otherwise i can just not see how.
How do people get overweight?
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