Nibbler appears to weigh a normal amount (seen when being picked up, etc.), but then takes a poop and his dark matter excrement is so dense that it has a colossal weight for its size. Where does all this extra weight come from???
Surely he should weigh at least as much as that all the time?
It's—because Nibblonians are bigger on the inside, and their digestive system is somehow able to either manipulate gravity or move its contents' mass through another dimension.
Excerpt form Nibblonians article on theinfosphere.org (emphasis mine)
[…] As waste, they excrete pellets of dark matter, an extremely heavy material formerly used as a starship fuel. The mechanism by which meat, which is made of ordinary matter, is converted into dark matter inside the Nibblonian advanced digestive tract is unknown, although it can be assume it may be based on a kind of black hole or pocket dimension which would explain their ability to eat much larger creatures whole and not visibly gain any weight. They are capable of swallowing themselves to enter their digestive systems and get out of a universe. The Nibblonian eyestalk is capable of emitting several types of energy that can wipe memories or burn skin.
Simple, matter inside a Nibblonian's digestive system does not interact with the Higgs field. Therefore it does not have any mass.
