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 Â Small sizes have one belly strap centered at the bottom. Example breed sizing: XXS Â Teacup; XS Â Westie; Small Â Beagle; Medium Â Border Collie; Large Â Labrador.