Hagrid was standing ready beside the motorbike, goggles on.
"Is this it? Is this Sirius's bike?"
"The very same," said Hagrid, beaming down at Harry. "An' the last time yeh was on it, Harry, I could fit yeh in one hand!"
This amazing machine was once owned by Sirius Black. It's huge, big enough to carry Hagrid (or perhaps it magically expands as necessary to accommodate its rider (PS1)). Later Arthur Weasley got hold of it, and customized it still further (DH4).