Don't get this thinking it's a backpack, think of it as a harness with pockets. You could definitely fit car keys, a phone, poop bags, etc in there, but probably nothing bigger than a standard 16.9 oz displosable water bottle. Otherwise, my pup was comfortable in it, it didn't restrict his movement at all, and he wasn't bothered by small items in the pockets. The pockets are small enough that filling one side didn't throw off the balance and cause the harness to sit unevenly on his back or pull to one side. Great color, too!