When you diet to lose weight you should decrese your overall body fat. For example if you yo-yo diet everytime you return to a high weight your percentage of total fat may have increased a little.
this is because, when you lose weight, most of the weight could be lean tissue, crash diets will make you lose weight for short term. But overall they increase weight long term as they focus on removing lean tissue, when you loss lean tissue its very hard to replace, instead of the tissue growing back fat is stored in the body instead.
So this means you’ll never be back at your old weight, your overall metabolism will have decreased, avoid crash diets at all costs and always exercise whilst slimming to keep you important try and keep as much muscle tissue as possible.