Before you make me a update, let me try to find some proof that Boleslaw was infact a Grand Duke and not just a Duke.
That will be hard.
The word used in Polish (... and the similar one in Russian and so on), "książę", can mean both "duke" and "prince". The usual - but by no means universal! - translation of this title as applied to independent rulers is "grand duke" if you want to differentiate it from dukes which are vassals of the person, else just "duke".
But that's just a translation. It isn't the actual title.