When looking for backpacks look for well known brand names such as: Camelbak, Deuter, Osprey, North face, etc etc. There are many good small sized packs out there to choose from those brands. Yes they can get a little hot and uncomfortable but it doesn't bother me. I have done 100 mile rides on hot summer days with a backpack weighing around 15 - 18 pounds.
One thing to avoid is messenger bags, because they are uncomfortable while carrying any amount of weight. In the cycling community messenger bags are just a fashion statement, with absolutely no practical advantages over a good backpack.