My small bag is the one I wear, with belt and shoulder strap. In it are only photo things, including lens cloth, lenses, memory cards, spare batteries, any filters I think might be useful.

The large back is a bit much on a trek but it includes a rocket blower, flash gun,  prescription meds if needed during that time.   I generally pack the bags depending on expected and possible unexpected conditions..which would inlcude aluminum foil, clothes pins, flash sync cord, perhaps a snack or two.  Most water, food, etc., I carry some other way.  Monopod is generally there somewhere, on the bag or in hand or over shoulder.

I would rather travel lightly as possible when on foot.