A January 2019 study published in the journal Animal Cognition, sought to answer the question of intelligence by asking pet owners to conduct a series of tests to determine their dog's mental abilities. One experiment measured a pup's "delayed memory" abilities by having an owner place a treat under a cup while their dog was watching. Then, the owner waited 60, 90, 120, or 150 seconds before letting the dog pick the cup with treats. Almost 2,000 dogs completed this test and the results showed that larger breeds were more accurate with their guesses over a longer period of time than small dogs.
So, what could explain this discrepancy in the intelligence between small and big dogs? One reason could be that big dogs have larger brains and therefore more neural connections. This means they're better equipped to process information and store memories. The results of the Animal Cognition study seem to support this hypothesis, with the researchers finding a "strong relationships between estimated absolute brain weight and breed differences in cognition."
Of course, that doesn't mean that your pint-sized chihuahua or 10-pound Yorkie isn't smart. There are two basic types of intelligence when it comes to dogs: adaptive and instinctive. Asking a dog to remember which cup holds the treat is an example of adaptive intelligence. Instinctive intelligence is what makes a dog a good example of its breed; it's why terriers excel at hunting small animals and bloodhounds are expert scent-trackers. So, even if your pup can't choose the right cup, it doesn't mean he or she is lacking in the brains department.
And ultimately, whether or not a dog is considered "smart" has nothing to do with how much you love him or her and vice versa. It just may be that your dog doesn't even think you're very smart. (And the cat would agree!).