Probably to due with less calories spent in manual work. Like walking instead of driving, washing clothes and utensils instead of using electronics, etc.
When your body needs 2600 calories per day just to stay at the same weight it is very easy to lose weight by cutting them to 2000 calories. When your body only needs 2000 calories then you need to cut them down to 1400 calories for the same result which may be impossible from a psychological perspective.
You need to eat about 7,700 calories too much to gain a kilogram of weight (https://www.livestrong.com/article/304137-how-many-calories-...). So, eating 25 calories too much each day gains you that kilogram in a year. If you keep doing that between ages 10 and 40, you ‘gain’ 30 kilograms, and chances are you end up obese.
You can counteract that by burning about 25 more calories each day. To do that by walking, you need to walk about half a mile extra each day (https://www.livescience.com/8931-calories-walking-burn-depen...)
At least as tracked by my Fitbit it can be upwards of 2000 calories difference, and 10-15k steps.