A&B cannot be more probable than A, but evidence may support A&B more than it supports A.
For example, suppose you have independent prior probabilities of 1⁄2 for A and for B. The prior probability of A&B is 1⁄4. If you are then told “A iff B,” the probability for A does not change but the probability of A&B goes up to 1⁄2.
The reason specific theories are better is not that they are more plausible, but that they contain more useful information.
Your scheme seems to be Jaynes’s Ap distribution, discussed on LW here.