This is a good question, and one that I don't have a complete or rigorous answer to. But I can describe my intuition: A theory is just a list of assertions. In

This is a good question, and one that I don't have a complete or rigorous answer to. But I can describe my intuition: A theory is just a list of assertions. In the absence of any evidence, each assertion is just as likely to be true as false. Therefore the probability of a theory with n assertions is 2^-n. So the more assertions there are, the less probability.