You have mentioned the magic word "slim". For fat people, the knees would need to support so much more load when they exercise. As one aged, the knees will also start to give way. To slim down through exercise, we need to reach a certain level of intensity before fat can be burnt (about 5km in less than 30 min at least 3 times a week) It is very difficult for a fat person to achieve that. Many fat people do exercise regularly but they will not be able to burn away their fat. Instead their knees will be damaged.
I have gone through it. It is real tough, requiring a combination of strict diet and exercise regime to remove the fat without hurting the knees. Most would not have the discipline. Furthermore, the process is a very long one. However, once the weight is reduced, maintaining is easy. It is the initial phase that is difficult. Can take about 2 to 3 years.