I have a 2L camelback that I've used during marathon training for a decade now. It has enough water capacity that on a long training run I finish drinking right as I finish my run, but not too much that I'm lugging around a bunch of extra unneeded weight. It straps close to my body so there's absolutely no jostling while I'm running, just a little bit of swishing of the water, but that's minimized by its slim but tall design.
It's got two zip pockets and one open pocket. I store all my running gels in one zip pocket and the trash in the open pocket. Then I store my phone, moleskin, bandaids, & keys in the second pocket. All in all, it's about as full as it can be, but it weighs <5lbs fully loaded.
Before I got this backpack I would drive out to various spots along my running route and plant a water bottle + gel. I always had to hope nobody had taken them and I'd be left at mile 16 with no water. It also meant significant forethought, because I had to map out my run and find good spots to put the water. I couldn't improvise day of, or I'd be stuck without my planned water breaks. Lastly, I had to drive out after the run and collect my trash.
Any larger bag would provide nothing of value for my goals and just force me to carry extra weight.