Depends on how you use the pack. If it will be a day pack to be used on side-trips while backpacking, there are stuff sacks for your sleeping bag that have shoulder straps - multiple us is good. If it is a dedicated day-pack I really see no need to worry about an ounce or two. A little too big is OK. Never know when you will need a bit more room.

If on the other hand, you trail run, then you need a pack that fits snugly and smaller is better. Even though heavier, trail runners and bicyclists seem to prefer Camelbacks.

You may consider getting a water bottle carrier that slips onto your belt (if you wear a belt) to carry the water bottle and just stuff the food in pockets.

For me a day-pack must carry more. I never go without rain gear and an extra insulating layer. I am pretty big on the idea of the "ten essentials". Day hikers who take too little are at more risk for hypothermia than backpackers who have their gear with them at all times.