Assuming that an increase in radioactivity is a side-effect of mass condensation, and that mass condensation is real and ongoing, large astronomic bodies like planets and stars will experience an increase in internal pressures over time.
This is because radioactivity results in an increase in the number of atoms in a given space. Where there was once only one atom, there are suddenly two. With more atoms occupying the same space, pressure builds up.
Some planet may stay unchanged in size for ever. However, others will eventually crack and expand due to internal pressures.
From evidence available to us, it appears that Earth stayed pretty much unchanged in size up until about 300 million years ago. Our planet had at that time a diameter roughly half of what it has today.
However, ever since then, our planet has expanded.
This is based on the fact that continental crusts are about 4000 million years old, while no ocean floor is older than 300 million years.
Also, if we cut away all the oceans on our planet, the continents fit perfectly together onto a sphere half the diameter of present day Earth.
Oceans are rifts produced by the expansion of our planet, while continents are the original crust.