This is Uruguay's dish. It is not from Argentina, or any other part of South America. When "chivito" is spoken about, it's understood or soon understood that this is Uruguay's property.
It's basically a steak sandwich - with ham and sometimes with an egg or other veggie goodies, along with cheese and then fries on the side. It's fabulous, and it the ingredients don't really vary too much from town to town or restaurant to restaurant.
And in case you couldn't tell, it is not on an individual's diet plan - if that diet plan is aimed at keeping a fit figure.