They can definitely poop that much in a day. I'd start weighing weekly, and try to get into a routine so its done at the same time of day and after the same series of events. For example: I always weigh in the afternoon on Mazuri days, after a long soak. This way, they tank up on Mazuri, but they drink and void their bowels in the soak. This makes for more consistent weights. Full or empty stomach and full or empty bowels can make a huge weight difference in a 28 pound tortoise. If your tortoise ate or drank less because of the weird overcast weather we've been having and then had a big bowel movement due to the rain, I would expect the weight to be lower.