Lowe Alpine make some quite large capacity waste belts, some are nearing 10L in capacity, which is certainly large enough to carry a full set of waterproofs, wind top, map and compass, and provisions for a day in the hills.
My personal favourites for Day sacs, are those manufactured by Camelbak, not because of their ability to store several litres of water, which in its self is important.
Ive got two Camelbak sacs, one a very old Cloudbreaker, that's done the three peaks challenge, three times, never on my shoulders but always borrowed by friends.
Its the materials used in their manufacture, their very high quality, and the stitching is very precise, almost as if their done by hand.
My Cloudbreaker is well over twenty years old, but looks new, and the nylon used for its construction, is made from a heavyweight ripstop material.
My favourite is the Camelbak Mule, which has more than enough capacity for a day out, and has one of the most comfortable and adjustable shoulder straps known to man.
Once its on your back, you simply have no idea its there, its really that cosy, and has four separate compartments, as well as smaller pockets for keys or spare change, and yet again, beautifully made.