Dog weights go up a lot with a very little increase in size, due to some very basic geometry.
- A dog with a chest span of 14 inches has a chest that takes up 15.59 square inches of space.
- A dog with a chest of 16 inches, however, is a dog that is taking up 20.37 square inches of space.
- A dog with an 18 inch chest circumference is a dog that is taking up 25.78 square inches of space in the pipe.
- A go-to-ground tunnel has an interiour space of 81 square inches!
- Barry Jones, professional terrierman to the Cotswold Foxhounds in Andovers Ford, and a former Chairman and President of the Fell and Moorland Working Terrier Club, and the founding Chairman of the National Working Terrier Federation, spanned an average of 300 foxes a year and said "I have not encountered a fox which could not be spanned at 14 inches circumference."
This is just square area. Cubic area gives you even more impressive numbers. For example, something that is one yard on each side (height, length, depth) is one cubic yard, but something that is 3 yards on each side is 27 cubic yards (3 by 3 by 3). The same thing happens with dogs; as height increases, so to does length and width, and these dimensions compound each other.
In the end, it is not weight or height that determines a terrier's ability to work so much as chest size -- and of course a strong dose of desire, a big shot of nose, and a willingness to use its voice.
No matter how much desire a dog has, however, it cannot overcome too large a chest size. Flexability has nothing to do with except at the margins. Nothing is more plastic than water, and yet you still cannot put a half gallon of water in a pint bottle.
A den pipe is anywhere from 10 to 40 feet long -- far too long for a dog to excavate except, perhaps, at a few tight spots. A dog that is digging a lot to get to the quarry arrives at his or her destination exhausted, oxygen depleted due to dirt pushed behind it, and unable to properly maneuver to avoid the slashing teeth of the quarry. It is a disaster waiting to happen.
Few dogs are too little or too smart to work, but many are too large and too dumb.