There is an entire thesis here (no, don't worry, I won't write it) on the materiality or immateriality of knowledge, which would have implications for pneumatology, consciousness, physics, eternalism, and so many other fields to begin to even list. Because if our heads did get heavier when we learned, and we could prove this came from added knowledge, then knowledge must have a material aspect to it and potentially a life outside of the brain. Out goes any hope of Cartesian dualism (though many would argue that it has none anyway). That would be a fun pseudo-thesis to write. But I won't, because I wouldn't want to weigh my head enough to prove the thesis. Data is so inconvenient sometimes.