The item in your link is pretty minimal. The smallest I have is a CamelBak HydroBak. No real room other than for water and maybe a couple of gel packs, small ring of keys... maybe a small wallet. But not all three. The only downside for me with it is that it doesn't have a lower strap, just a chest strap. As such, it flies up and hits the back of my head on jumps. For that reason, I've been eyeing the USWE Outlander 2 MattMay mentioned.
Just picked one up and excited to try it out. Been using a Raptor 14 but wanted to reduce the weight on my back and try out a hip pack. The Evoc Pro has by far the best waist belt of all the options I've seen.I picked up an evoc hip pack pro and took the bladder out so I could carry my dslr in the thing and a spare bottle or two. I wasn't sure about it when I first put it on but once I got on the trail I didn't even know it was there.