It's not exactly easy to explain. Richie himself only holds a fraction of the Nova force, which is spread amongst the entire Nova Corps. So normally he's roughly as powerful as a lot of more powerful Avengers members, but he isn't way above their level. The Nova Corps is a galactic-level entity though, so they're a lot higher up the cosmic food chain than the Avengers even if any individual centurion is not significantly more powerful than an Avenger. Think FBI vs city cop.
However, Richie did briefly hold the entirety of the Nova Force during the Annihilation comic event when he was the sole remaining Nova centurion, which made him effectively one of the more powerful beings in the universe at that time (not counting things like eternals and celestials). However, that was temporary, and once the Corps was reformed the Nova Force was again spread out among a large number of centurions.
EDIT: Actually, think of him as like Green Lantern compared to the rest of the Justice League, if Green Lantern wasn't a member himself. That's not a bad comparison tbh.