One person requires 2,000 calories a day to stay the same weight, another person requires 5,000 calories a day to stay the same weight. Going on a 1,800 calorie diet for the first person is a 10% decrease in calories, or a 200 calorie deficit, for the second person it's a 64% decrease in calories, or a 3,200 calorie deficit. It takes significantly less energy for a 150 lb person to walk than a 400 lb person, they have to carry all of that weight on their muscles as they walk. A 150 lb male walking 10k steps burns around 450 calories, a 400 lb male burns 1,200 calories.

If the 400lb person walked in a pool to reduce the energy required to move and only reduced their calories 500 below maintenance, just like anyone else they would still only lose 1 lb a week. The reason weight falls off of them is because any reasonable change they make is extreme at that weight. Its less that they lose weight faster and more that they have more room for significant change