Which bike is Hagrid riding in the first Harry Potter film?

The bike that Hagrid rides in the first Harry Potter movie, The Sorcerer's Stone, is a 1959 Triumph Bonneville. In later movies, like the Deathly Hallows, it is a different bike that Hagrid rides.

I am not sure why there was a swtich in the type of bike that Hagrid rode. Maybe there was a need for the bike in Deathly Hallows to ride faster or maybe due to the growing size of the characters, they needed to find a bigger bike.