Not all dog harnesses are created equal. Some go straight across the chest which is the equivalent of a person walking with a rubber band going across their chest and holding their arms down (pictured below).
This isn't comfortable and it restricts their natural walking stride.
A dog harness that cuts across the front of the chest, drawing a straight line from the chest to the harness stops the dog's front limbs from being able to move forward in their natural stride. Thanks to the most comprehensive dog movement study completed in 2011 by Prof. Dr. Martin S. Fischer at the University of Jena in Germany we now know a dog's movement is restricted by this style of harness because the centre of rotation of a dog's front limbs is at the top of the scapula.
The "Jena Study" took 4 years to complete and had a total of 327 dogs from 32 breeds that varied widely in body size, and in the angulation of their joints and frames. It was found that it was reasonable to conclude that all dogs, regardless of size, walk similarly. This means regardless of the size of your dog, you should be looking for a harness for your dog that doesn't cut across their chest, drawing a straight line from the chest to the harness (like the below photo on the left).
The correct type of harness all dogs should be wearing shouldn't restrict the natural movement of their front limbs. Look for a harness that is more parallel to their collar. Instead of cutting across a dog's chest and attaching to the harness, the front strap should run from their chest towards their shoulder blades (like the above photo on the right).