Artic Tractor Axle configurations

In the US all artic tractor have 2 drive axles. Here our truck are heavier and have only on drive axle, why is this?

Also there are loads of axle configurations. The merchant that hauls our grain has some with the centre axle driving and a full sized rear axle, some with the rear axle driven but a full sized centre axle and some a driven rear and the centre axle with very small wheels. What's the story?