Assuming that an increase in radioactivity is a side-effect of mass condensation, and that mass condensation is real and ongoing, all astronomic bodies 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.
A planet may stay unchanged in size for a very long time. However, at some point, the internal pressures will become so great that it will crack and start to expand.
From evidence available to us, it appears that our own planet 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.
Keeping in mind that mass condensation, discussed in earlier chapters, is something that increases both mass and radioactivity, we now have a way to explain both the size of the dinosaurs and the expansion of our planet.