Gotbacz, as you pointed out yourself earlier CR is a flimsy thing. Trolls are just generally powerful for their CR. But compare a barbarian hill giant to some CR 9 dragon and your hill giant seems funny.
Not that much. A Barbarian Hill Giant would have exactly the same hp and AC as a Young Red Dragon, and two attacks which would both hit AC 24 more frequently than YRD with its secondary attacks. Sure, there are dragon immunities and breath weapon to consider but the difference is not terrific. I would prolly rate the BarbHillGiant one CR lower than the Dragon, but I would absolutely consider it more dangerous than a Giant Flytrap or a Brachiosaurus. So yeah, CR is a ballpark, but as long as you're within the ballpark, it's fine.