I like to go for 3-4 hour hikes in the desert, which means 3-4 liters of water. This perfectly fits two 1.5-liter Nalgene bottles in the main pocket + two 0.5-liter Nalgene bottles in the two side water bottle holders. The little pocket then fits my wallet, keys, some granola bars, a backup battery for my phone, and electrolyte powder. In the main pocket above the water bottles there's still room for a small first aid kit. Before, when I put all this in a regular sized backpack, the water bottles shifted around and became uncomfortable as they moved to annoying places on my back or fell over sideways because there was too much room.
The small size also allows air to get to my back more, drying the sweat there and cooling me off. The straps are comfortable, and the cross-chest strap is perfect and adjustable. I absolutely love this backpack.