Ragnos and Kun Quotes

“Marka Ragnos ruled the galaxy with an iron fist. He was the Dark Lord of the Sith — the most powerful of the most powerful. But now he is dead.”

Source: Tales of the Jedi: The Golden Age of the Sith 2: Funeral for a Dark Lord (Publisher's Summary)

“The most powerful and dangerous of all Sith Lords, Exar Kun was once the apprentice of the Jedi Master Vodo Siosk-Baas.”

- The Official Star Wars Fact File

How does one understand the meaning behind these quotes?

How do we know that Exar > everyone before him, or if he > everyone during his era?

Same for Ragnos.

Can we?

I would take statements like these with a grain of salt. No we can't, but I'd say Vitiate is more powerful than both

Ragnos>Kun.