adipose tissue can hold more than 30% of its mass in water.  500 pounder say 200 lbs lean body mass, 300lbs fat.  .3x300=~90lbs of water, which means at any given time based on sodium consumption and water balance this fat probably has 30-50 lbs of water retention (dehydration from soda and salt = more water retention).  This means that super morbid obeasts can rapidly change weight (within this range) whenever they make seemingly minor changes to their diet.  Their actual body mass can also change more rapidly because everything is a proportion;  1% of 150lbs = 1.5lbs, 1% of 500lbs = 5lbs.