I have several bags. Too many, really. None of them are perfect, but the one I end up carrying most often is one which seems to meet your requirements: small(ish), lightweight, enough space for a camera and a couple of lenses. As an added bonus, it expands by unzipping in the back to occasionally carry a larger lens in it's case (back area has no padding), like the 60-250mm I use.
Lowepro Passport Sling III (Black) LP36657 B&H Photo Video
It is listed as: 5.7 x 17.3 x 16.1" / 14.5 x 44.0 x 41.0 cm in size, but it really isn't that huge. The main case is about 8-9" in height and the expandable back portion comes up the strap to get to near 16". (see the illustration at B&H) Expanded, that area can hold all kinds of various items you may need, including another shirt for layering or various photography supplies. Or leave it zipped for a small bag. There's a water bottle pocket in front.
What I guess I like most is that everything is available at one level right at my fingertips on my side, so I can reach in, grab something else and make a change. Or if walking a long distance, the smallish size, light weight, and wide strap means it's not a burden. I've worn it all day.
happy hunting!