The B-52 is another example of the production aircraft having a smaller fin than the prototype.
Though I completely agree with the observation, I can also say that some aircraft have fin changes (addition of dorsal fin, in particular) to change the behaviour of the rudder. I understand that in some cases (Cessna 180 to 185 for example) adding area to the vertical tail would not have helped, as much as making what is already there work with greater effectivness.
Another fairly obvious reason, is the addition of floats to a landplane, where the additional keel effect forward of the C of G results in a need to balance the keel effect by adding a ventral fin, finlets.
Pilot DAR