One thing I can tell you from my very experience is, you can have 20 physicians in a room and still won't find 2 to agree on anything. So evidence shows that to decrease the size of the liver, there is no consensus on how long the diet should go on for. With bypass, given they have to access the gastro-esophageal junction which the left lobe of the liver can obscure, this will definitely warrant a diet of longer duration. In the paper I read, generally the longer the diet the more shrinkage, however... A greater decrease in size was noted with higher protein diets. Also you risk being in a catabolic state (this is not good for healing) if you are on these low calorie liquid diets for too long.
All this to say, some surgeons change their practices to reflect the evidence. Some were just trained a certain way and stuck with it.
Im on a week long diet and my BMI is 41