A six pound weight loss on a 600 lb obeast is one bowel movement and possible dehydration from traveling. That doesn't represent actual weight loss - the only significance of the number is that she didn't GAIN 30 to 40 lbs per month as usual. With only minor changes, a 640 lb humanoid should be able to easily drop lots of weight simply due to the caloric expense of keeping that much flesh alive on its carcass.