How radioactivity changes with size
Radiation can be absorbed internally
The total amount of radioactivity of a given isotope is directly proportional to the number of undecayed nuclei you have. If you ignore the mass of any radiation emitted then doubling the mass exactly doubles the activity.
But radiation can be emitted at any point in the material.
Alpha particles that are emitted in the middle of a sample will just be absorbed by the sample and can’t be detected. Only those particles emitted near the surface of a sample make it to the outside world.
Animation showing how alpha radiation is often absorbed by the source itself and never makes it into the environment to be detected.GCSE Maths exam soon?
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So for a large sample of an alpha emitter the total amount of detected radiation depends on surface area more than volume. In we can ignore this effect because the size of a laboratory sample typically stays constant.
In our animations we tend to show particles being emitted anywhere on a single face.