Units of volume per units of weight, other things being equal, are greater in larger packs so I stick with my larger pack.

I like having the larger pack when unexpected things happen (e.g. having to carry a 5 gallon bucket inside the pack on a beach hike, carrying floats and ropes home which I found on the beach, carrying a friends gear because they are tired or hurt, etc.)

I also like the packing convenience of a larger pack. No need to stuff everything perfectly. Just throw it in as you would in the back of a pick-up.