Too small, and you'll wind up lashing gear to the outside, which can throw you off balance. That means multidensity or compression-molded foams in the hipbelt and shoulder straps and a suspension system that distributes the weight evenly. The bag itself also contributes to the way a pack carries. Compression straps cinch the contents and eliminate sagging, and load-control straps keep the pack close to your body. External-frame packs carry weight just fine but don't flex much when things get seismic.
Capacity goes up, and flexibility begins to drop.
This helps center the load over your feet, where it belongs. The many layers of foam. The desired effects are stability and comfort. Stability is achieved by placing most of the weight on your hips. A stable load must also ride close to your back.