Dynamics of planetary size What happens when a planet becomes massive? Already in Jupiter, one sees self-compression, where size grows sublinearly with increased mass. At 70x Jupiter, heavy isotope formation creates brown dwarfs, i.e. energy producing planets; at 80x Jupiter, hydrogen burning creates star-like behavior. The sweet spot for planetary size appears to be "puffy planets", about half the mass of Jupiter, but twice the size.

https://www.forbes.com/sites/startswithabang/2017/04/24/whats-the-largest-planet-in-the-universe/#6011f65275c6