You are feeding your body down to the cellular level highly concentrated nutrients and as such your body is willing to give up hoarding the extra fat. Bye bye fat! In addition, the juices are much lower in fat calories than the processed foods and meats etc we usually eat.
The heavier a person is, the faster the weightloss will be. The thinner a person is, the slower it will drop. I've known people to drop four and five pounds a day in the beginning of a fast and they'll slow down to one or two pounds a day towards the end.
Enjoy the juices, they're life's elixirs!