A dog's weight to height ratio can be used as a measure of their athletic ability. You take their weight in pounds and divide by their height at the withers in inches. Metric Conversions The closer the figure to 1, the more athletic the dog's structure. A figure of 4 or more places the dog in the 'at risk' category for agility and other impact activities.
I think the important thing to take from the measurements is that if your dog is a breed that is tall and lightly built, it is going to be more athletic than one that is short with a heavy build, and is going to be able to maintain their performance for longer; and a lean dog will have an easier time in agility than an overweight dog.
Thommo is 2.9
Dusty is 2.0
Canine Fitness - Chris Zink