Editing Openai/6946e998-79f0-8007-9a65-19828450838d (section)

=== Planet mass determines whether the planet can: ===
* hold a thick atmosphere against escape
* keep a molten core long enough to sustain a magnetic dynamo
* maintain vigorous mantle convection
* recycle volatiles through volcanism and subduction
* allow long-term internal heating

Small planets (Mars) lose heat fast, lose magnetic fields early, and lose atmosphere through solar-wind stripping.
Large planets (Earth, Venus) retain heat, retain atmospheres, and sustain volcanism.

So, yes, the root control is size.
But size expresses itself through atmospheric loss, tectonic viability, and internal heat flux.