These are all generalizations because nothing is written in stone. First I would ask if you are referring to actual intelligence or trainability? They are 2 entirely different things. Dogs from the working and herding breeds are usually the most intelligent and trainable, they include large and small dogs but mostly larger breeds. Course there are some dumb ones in there too, huskies are not noted for either as a rule. I have always had bouviers and akitas, both extremely intelligent but tough as nails to train. They have a thought process that makes a lot of humans look dumb in comparison. Terriers were bred to go to ground (terra) after critters that could easily kill them. They are courageous and tenacious but you don’t see them in obedience trials because they are not overly smart or trainable. Poodles are usually smart and trainable, the toys being the least of the 3. Hunting dogs are usually in the same catagory as terriers, they are bred for a purpose and some still are good at that purpose. And some are extremely eager to please which makes them trainable.
Dogs are not unlike people, there are no hard and fast rules. My dingo mutt is so smart it is scary, she definitely thinks and obeys when she chooses. I go back to my usual mantra, go to a reputable breeder and question them about their breed. Someone who has been in the breed for 20 years or so. They can give you the best lowdown on their breed.
One should also say that dogs are like humans in that some are smarter than others. I know a couple of people who have utility degrees on their afghans but they bred the dogs themselves specifically for obedience whereas most afghans are dumber than dirt and not particularly trainable either.
The link @hiphiphopflipflapflop is good and explains the different types of intelligence more eloquently than I. It is old though, JRT’s have beenrecognized by the AKC for some time now I believe.