This is what the site says:
1" Leather Dog Collars
These collars generally work well for medium to large breed dogs. The double layer, lined collars in this section are pretty heavy duty and are designed for stronger dogs over 50 lbs.
1.25" Leather Dog Collars
These collars are designed for large to giant breed dogs or for anyone who is looking for a wider collar than the standard 1" width. The double layered, lined collars in this section are much thicker and heavier than those you'd normally find in pet stores and are designed for dogs who need a stronger collar.
I like how this particular style is double layered.