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.