I live in Argentina and I travel a lot with Latam.
After some recent changes on fare tickets on South America, people get very low prices if they travel with cabin luggage only. This means that the carry-on suitcase number increased by 100%.
A plane has limited space for carry-on suitcases stored in the overhead.
So now, Latam personnel controls carry-on suitcases (size and weight) at the gates while passengers are boarding the plane. You may get your suitcase (or big bag) not matching the size being checked. They might or might not apply extra charges.
Even if your carry-on matches the size and the max. weight, if there's no space for it overhead they will check it. On these cases, they will not charge you.
This applies at least to Argentina, Chile, and Peru. Not sure if Brazil has the same problem.
 As additional advice, I suggest you put a plastic bag on the top of your carry-on. If they decide to check this suitcase, you can quickly recover and keep with you any valuable stuff (either expensive, fragile or priority use).