What about more lowly populated plains and jungle east of the Andes? Could the western border of Brazil be farther west in Paraguay, Bolivia, eastern Peru, etc.?
There is no stimulus to do that apart from "Hey let's seize some land to make Brazil bigger". Brazil has more than enought agricultural and pastoral land, making those plains useless. Brazil also would need to keep a garrison there and try to populate it and to integrate the local minorities, it would be a drain for the economy.