Humans are not the greatest at smelling. Compared to animals like rats, our smell systems are very small. Nevertheless, we're still decent at smelling!
In one experiment, human participants were blindfolded. They were then asked to smell their way across a scented field. It turned out that the participants did this task quite well!
Our sense of smell probably developed for different reasons than rats. For example, we use smell to detect differences rather than to mark territory.
So, while all brains come in different shapes and sizes, it’s important not to judge a brain region by its size.