This is a good point. And that's why Fringe lost me (I'm told to my loss for good character development, but oh well) in the very first episode. It wasn't so much that the amazing growing baby violated conservation of mass in itself, but that the mad scientist and his son who was supposed to have a good basic understanding of science didn't even seem to *acknowledge or notice* that where the mass came from was an issue or come up with a technobabble explanation or handwave.