At Bibi's Boutique, we sell a lot of harnesses. We manufacture in two different ways - as a "step-in" and in a more "halter" style. We are big fans of the harnesses for the following reasons; step-ins are easy to put on - you can lay the harness on the floor, train your dog to "step-in", gather the two ends, clip them together, and attach the leash to the two metal rings. Nothing easier. The pressure is on the chest if the dog should pull, and it is very comfortable to walk with, and at Bibi's, we make lots of color-coordinated leashes and harnesses that are very nice-looking.
The other type of harness is put over the head, with a paw inserted in one side. We make these in leather. One strap circles the girth, and is buckled. The leather is soft and lined with a rubbery Nubuck so it is very comfortable on. This harness is very safe for dogs as they cannot slip out of them when properly fastened (whereas with the step-in, some dogs learn to shrug the shoulders to get out of them).
The goal of the harness is to stop pulling on the collar, i.e. the neck. Many of the little breeds cannot tolerate the constant pressure on the delicate trachea from wearing a collar and being pulled on a leash. The harnesses are made to avoid the trachea and throat area completely.
Little dogs can develop a terrible hacking cough due to damage to the trachea from wearing collars. It is imperative to wear a harness to avoid the damage to the neck area, mis-alignment of the spine,
choking, and trachea breakdown. This is our strong recommendation.
As for the big dogs - they are very strong with muscular necks and large chests that are empowered by the harness - fine for pulling a dog sled, but in general, one has better control over the dog with a staunch collar strap and a solid leash that communicates a firm hold, rather than a harness that allows a dog to gather power and use it by pulling with his chest and digging in with feet and haunches that add more strength to the harness. This being said, I would prefer to see a big dog with a harness rather that any type of choke chain or prong collars for the reasons stated above.
This is this reason why your little dog should be wearing a harness and your big dog shouldn't.