Twice the source mean radius; body diameter only, excluding rings.
Saturn is often recognized by the space its rings occupy. Here the disk represents the planet’s body only. Leaving the rings out makes the numerical diameter and the visible outline describe the same thing.
Compare like with like
Put Saturn beside Jupiter to compare two planetary bodies. Then try Earth. The scene does not use the outer edge of the ring system as Saturn’s diameter; that would answer a different question. The displayed value is a volume-equivalent mean diameter.