Q: Is the crust getting thinner or is the mantle getting smaller
The crust is actually getting thicker very gradually as the Earth cools down. This process is well seen on Mars, where due to its smaller size and greater distance from the sun, Mars is cooling faster than the Earth. No one knows exactly how thick the Martian crust is, but what we do know is that magma can no longer find its way to the surface. All the Martian volcanoes are extinct.

On Earth, the magma ejected from volcanoes is mostly recycled oceanic crust. Sea floor spreading from the mid-ocean ridge pushes the oceanic crust on the Pacific Plate toward the continental crust on,for example, the North American plate, where it is forced to dive  beneath the continental crust and driven down into the mantle. For a number of reasons (friction, water content etc) this causes it to melt, and being lighter and more fluid than the mantle it rises to the surface, where it escapes through volcanism.

This is not the whole story, because if oceanic crust is disappearing as it is pushed below the continental crust of another plate, it must somehow be replaced. This happens at the mid ocean ridge, where a deep chasm plunges through the comparatively thin oceanic crust, enabling fresh magma from the mantle to rise up and create new ocean floor. This is constantly spreading out in opposite directions from the mid ocean ridge, hence the term 'sea floor spreading'.