Look around at what others in your situation are doing if you can. What breed are those chickens? Are they indigenous chickens that might be smaller than you expect? Many landrace chickens adopted to local conditions are relatively small compared to certain dual purpose breeds.
How are you managing them? Are they allowed to free range or do you have them locked up so they are totally dependent on what you feed them? How good is your forage? For thousands of years chickens have been kept where they free range and feed themselves by foraging. I grew up on one of those farms. The chickens were a barnyard mix, fairly small as barnyard mutts often are.
There is a lot I don't know about your situation, your goals, set-up, or management techniques. There are issues to managing them as many people have managed them over thousands of years, quality of forage and potential predator risk for example. I did not see anything alarming in your description of how the previous owners managed and fed them, sounds pretty familiar.
It sounds like you want to change how they were managed. How convenient is that feed store? What types of feed do they carry? You might chat with them to see if they can get what you might want.
Are you sure? Are they hiding a nest on you? As you said you don't know how old they are but in Uruguay you should be at the height of the egg laying season.
