There are a few things that might be confusing you:
* Strength: The Strength HM lets you move large rocks, but those are different from boulders. You won't be able to move the large boulders you see around the world with Strength.
* Rock Smash: This HM lets you break smaller rocks, but it won't move boulders either.
There are no in-game mechanics for moving large boulders in Pokémon Platinum. If you're trying to do something specific, please clarify what you're aiming for and I can try to help!