Why are some breeds of dog so big and some so small? Humans aren't so extreme (pygmies & basketball players)?

Humans could of been if they'd been selectively bred that way.

From evidence of remains found it seems that there have been races of giants and races of pygmy people in the world.

The difference in dogs is that people have selectively bred over generations and generations of dogs to make the breeds we have.

Different DNA's

We selectively bred them that way. As previously stated, humans could also have that kind of weight and height variance if we'd selectively bred ourselves for such things, but we haven't.

breeding that started thousands of years ago

...you're serious? You don't know?

Do you think there's Golden Retrievers and Chihuahuas running around in the wild?

Humans BRED THEM to be the size that they are.

Jesus Christ.

Selective breeding. Larger dogs were bred together to create a "breed" that is larger and more muscular than their ancestors, the wolves. To make smaller breeds, puppies were often kept in small boxes to prevent them from growing. Similar methods were used in creating dogs with similar shape, size and disposition, and those dogs were breed together to create breeds. Some were even cross bred to create more breeds. Different climates forced the dogs to adapt and breeding between other canines (such as wolves and coyotes) created dogs with longer, shorter, thinner or thicker coats. Some dogs were "designed" especially for one job, such as Olde English Bulldogges, which were bred so that they were shorter and muscular, with a shorter muzzle and under bite to help it latch onto the bulls. The wrinkles were designed so that the blood didn't get into their nose and affect their breathing. Dachshunds were originally bred to flush out prey and they had crooked legs to help them dig and get into smaller spaces more easily. Their tails were made so that the hunter can grab it and pull them out if they needed to, and they had vibrant colours to help the hunters tell the dog from the prey and long ears to help the scent better travel to their noses.

I cant give u an exact answer but this is what I know:
all dogs have been modified by humans for specific purposes for examples terriers are small and naturally aggressive towards small animals (rats, mice etc) so its obvious they were bred for homes to catch rodents, or salukis bred to run or foxhounds for their nose and endurance.
Each breed has characteristics that come from hundreds of years of breeding to get them were they are today. People have bred dogs to suite needs for whatever purpose- hunting, herding etc etc.
Hope that helped a little.