Why the Australian Tomahawk steak is larger then US? If you do a search for Tomahawk steak on Google Image, you will see two very different Tomahawk steak, the US ones, some simply referred it to Rib Eye Bone-In French Cut, are much smaller, with smaller bones at the tail and much more elegant looking. 
While the proper Australian Tomahawk is so much larger in pic, and weight in 1.8Kg+.

What are the different between these two parts? Are they even made with the same cuts?
They are the same cut.

Quoted from Meat 102: Cuts, Anatomy &amp; Preparation (emphasis mine):


  Some restaurants will call the ribeye a “tomahawk steak” if the entire rib bone is left on and french cut by the butcher, since it then looks like a small hatchet.  It is common for butchers to cut the bone down a bit, however, for packaging purposes ... Sizes and portions range from anywhere between 14oz to 36oz, most often hovering around the 20oz mark in restaurants. 


36 oz is about 1 kg, or half as large as the "Australian Tomahawk" -- which can be attributed to having the full rib bone, as well as probably a thicker cut (and a generous pinch of marketing-speak and publicity).


  The Tomahawk is one of largest steaks in Australia, weighing in at
  1.3kg – 3.6kg (1.8kg average). Each Tomahawk is a rib-eye, or scotch fillet on the bone...


(see also StockJournal article)

EDIT: It's also called a cowboy steak or cowboy chop. So, the uncut rib bone seems to be the primary difference between "Australian" and "US" tomahawk steak.