I would guesstimate that most people in the developed world consume at least 1000 kcal more per day than they spend.
While I can't confirm that the 1000 kcal fact is true, it is fairly common knowledge that the United States of America has an obesity epidemic. Any quick search can show you hundreds of charts and figures. I won't argue that we're overeating. Personally, I'm from Canada and while it isn't quite as prevalent here, people still overeat.
If calories-in-calories-out is correct, this overeating should lead to 2lb (1kg) of fat gained per week. However, the average person only gains about this much per year in his or her 20s and 30s. Why not much more?
If you think about it, this is all about equilibrium. I currently weigh a modest 200 lbs. If I put on 100 lbs by overeating, I wouldn't necessarily gain 100 lbs of fat. My muscles would build up in order to move my body around (assuming I wish to stay mobile). As such, my TDEE would increase. At some point, my mass, the muscles I've gained to move my mass, and the number of calories I enjoy intaking will all balance, and I would stop gaining weight.
If someone were to consistently gain 2 lbs per week, they would need to eat 1000 kcal more than their TDEE. With 100 lbs of weight gain, my TDEE could change as much as 600 calories meaning than I now have to eat 1600 calories more than before. At 200 lbs I need to eat 2200 calories more, 300 lbs I need 3000 calories more, etc.
This scholarly article that shows that obese individuals have higher muscle activation even though overall muscular strength is reduced. Articles like this prove that we can't assume 100 lbs of weight gain is 100% fat.
For TDEE comparison let's assume for every 100 lbs of weight gain, 25% is muscle mass.
Random statistic: Male, 30, 6', 200 lb, Sedentary, 15% BF (i.e. 30 lb fat to start).
200 lb @ BF 15% = 2443 kcal
300 lb @ BF 35% ((30 + 75)/300)*100 = 2737 kcal (+294 kcal)
400 lb @ BF 45% ((30 + 150)/400)*100 = 3031 kcal (+294 kcal)
500 lb @ BF 51% ((30 + 225)/400)*100 = 3325 kcal (+294 kcal)
That's theoretically enough to add an extra 31 pounds to each person every year.
I think this is where you might be getting confused because this is a misleading statistic. The source states that we're overeating by 304 calories a day. Which then finds that:
(304 calories/day * 365 days/year) / (~3500 calories/lb fat) = 31.7 lb fat/year
This is theoretical. In reality, if everyone in America put on 31.7 lbs of fat this year, we would no longer be overeating by 304 calories per day for our new weight. Now we'd only be overeating by 200 calories per day, so, we'd gain 20.8 pounds in the second year.
So, to be clear, 31 lbs/year is misleading. It would be 31 lbs in the first year.
Additionally, the source states that the average diet used to be 1850 kcal a day twenty-five years ago. With 304 kcal added to that, we're now at a daily consumption of 2154 calories on average. The gentleman in my example would be losing weight at this level of eating. It's not that everyone is putting on fat every day, it's that America as a whole is just heavier already. That's why you see the weight being gained slowly because most people, on average, are already eating in the 2100 calories range (which includes that 304 increase) instead of the 1850 range.