What do we know for a fact:
Sirius owns a motorcycle that can fly
Sirius gives motorcycle to Hagrid
The motorcycle is described as big, big enough to fit a half giant Hagrid.
What we can safely assume:
Motorcycle was muggle-manufactured.
Sirius enchanted the motorcycle himself.
What assumptions are less safe, but still very reasonable:
The motorcycle was manufactured at a normal size.
Sirius being a normal sized human would ride uncomfortably in an over-large bike.
What conclusions these could lead us to.
Sirius upon handing the bike over to Hagrid the night of Lily and James' deaths, enlarges it for Hagrid.
Hagrid himself enlarges the bike after being given it.
The bike itself was enchanted to fit the rider (similar to both the Weasleys' car and the Ministry's cars).
Canon info to further our ideas:
Cars are shown to be magically enhanced
a. motorcycle can fly
b. Weasleys' car can fly, become invisible, and has some enchantment to enlarge the cabin to fit more people/luggage.
c. the Ministry owns cars that are magically enchanted to also have more room than physically possible.
Sirius is an accomplished wizard, probably similar in skill or better than M. Weasley.
Hagrid has been shown to be able to enlarge things magically, like his pumpkins.
My belief.
The bike itself was enchanted to fit the rider.
This fits best with other enchantments we have seen on other cars, where the car would magically enlarge to fit the riders and luggage.