This is one explanation I found. I am not sure about it. I also found a u of cal article saying it's not proven to shrink the liver. I have found a lot of articles saying that the liver being enlarged is a huge issue if you are having a sleeve. I guess it's better safe than sorry but I am still going to look for a better study or proof.
Some surgeons mandate a very restricted diet - one that is low in fat, sugar and carbohydrates - for one week prior to surgery and even if your surgeon doesn't ask you to follow this diet, doing so will increase your chances of your operation being completed laparoscopically because it will shrink your liver down in size. By following a strict diet, your body reduces its glycogen stores (glycogen is a form of sugar stored in the liver and muscles for energy). With each ounce of glycogen, the body stores 3–4 ounces of water, so when you follow a very strict diet, especially one that is low in starch and sugar, your body loses its glycogen stores and some water. The liver shrinks as it has less glycogen and water in it. This diet is only recommended for one week before surgery and is not to be followed post–operatively. It is quite possible you lose a lot of weight that week, but it will be mainly be water loss. Please ensure you continue to drink throughout the day.