I do a lot of day hiking In high elevation, most backpacks made for dogs to fit in don’t have space for personal items or water bladders.  with this backpack I can pack necessities for myself for my dogs a water bladder and my dog still fits comfortably. I’ve got a 7 pound Chihuahua and 15 pound Chihuahua mix fit in comfortably, not together, of course, the only complaint I have is I wish the hip straps were more solid/padded and could bear more weight because the area of the bag the dog fits is out of the center of gravity and it adds more weight to the shoulders. With that being said I’m still beyond impressed how thoughtful the design is. 
Thank you