The differences between the Day Packs include the materials and design.
The Ultra-Sil Day Pack is made from 30D Ultra-Sil Cordura nylon and includes adjustable shoulder straps and a daisy-chain webbing loop (great for bike lights). It has a two-way zipper closure with a generous opening and weighs 2.5oz/72g.
The Ultra-Sil Nano Day Pack is made from 15D Ultra-Sil and packs down to the size of an egg that weighs 1oz/30g. With an asymmetrical zipper, lightweight fabric, and slightly smaller capacity (18L vs 20L), the Nano is a great option for those that need to save on weight and space.