Why is she standing on a Saturn the size of the Sun? Why Saturn the size of the sun?

Sana is the size of the sun in her real form. Bigger actually. She stated as much.

She basically SCP-179

Keo said:

Sana is the size of the sun in her real form. Bigger actually. She stated as much.

She basically SCP-179

Exactly. Saturn should not be this big since it would actually be smaller than her breasts.

Earth should also not be this big, but Sana still has her limiter on her head. You could easily explain all of this by assuming that the limiter isn't a binary on/off device and can be tuned to different settings. The ratio between Earth and Saturn seems about right at a glance.