There are, however, some smaller packs that should work, and I suppose this is what you're after. Lumbar (fanny) packs are usually the best option; they're held low on your body, of course, so don't hop around much. And many have room for water bottles in addition to extra clothing and the like. The Lowe Peak Runner ($40) is one example, with 400 cubic inches of capacity and a design that pulls tightly against your body. Mountainsmith has always made superb lumbar packs, and their Blaze ($80) is nearly twice the size of the Peak Runner. It has wide straps that really keep it stable, and two water bottle holders in addition to the internal space.
These days, some pack makers are manufacturing lumbar packs that also have shoulder straps, so the load stays low but doesn't bounce. The North Face Backwater is such a pack (it's $90). As a bonus, the shoulder straps make it easier to attach a hydration tube so it's close to your mouth. Not as big as the Blaze, but with 500 cubic inches it should have enough room for your extra gear.