Things are not the same for all kinds of planets. A rocky planet the size of Mercury is going to cool off much faster than one the size of Earth. This is largely due to the difference in volume to surface area ratios and related effects. A more massive planet will have a much higher amount of heat left over from formation, and it will lose that heat slower. Plus, it will have a higher quantity of radioactive elements relative to its surface area. This is why the Moon is geologically inactive while the Earth still is, despite being made of mostly the same stuff.

Once you scale up to gas giants these factors become even more important. Those planets retain a crap ton of heat from their initial formation and are comparatively inefficient at radiating it away into space. More so, they continue to generate heat through ongoing settling (raining of Helium down to lower levels of the atmosphere, for example).