Expanding Earth: Moving continents and science

To explain why some continents that are now oceans apart seem to have once fit together, scientists hypothesized that the Earth itself must be dynamic. 

More precisely they thought it was expanding over time, slowly inflating like a balloon, pushing continents apart and creating oceans in between.

This theory was momentarily popular but cracks soon started showing. Why was it expanding? Was it getting heavier too? Did it affect gravity? And if this was really how planets worked, why wasn’t Mars or Venus doing the same thing?