New Extreme Packs - Ergonomic, Waterproof and Tough! Reflective Trim + Stripe. Interior mesh for comfort. Waterproof zippers. Waterproof nylon. Large and Medium size bags have two belly straps for comfort and stability. XXS Small sizes have one belly strap centered at the bottom. Example breed sizing: XXS Teacup; XS Westie; Small Beagle; Medium Border Collie; Large Labrador.