I'm not trying to be mean here.
If you put a 100 gram rat into your snake, your snake will weigh 100 grams more than it did prior to feeding.
Until it poops, it will weigh 100 grams more.
Once it poops, it will then be empty. That weight should be more than it's prior weight before eating the 100 gram rat, but less than 100 grams.
I may not be writing it clearly, but I think that should be understandable.