I have a Giant road bike, carried 2 large bottles of water in cages, large as I could find seat bag. In it I carried 2 tubes, tire levers, tools,camera, cash, pump on frame, clip in road shoes, pair of sandals tied on, plus some stuff in 3 bike shirt pockets.
My partner on a Trek road bike also carried 2 water bottles in cages, 1 liter camelback, road shoes, sandals, 2 tubes, pump, tire irons, sunscreen, extra towel for drying after her numerous swims, camera, blackberry, cash, some food.
We both had 23x700 tires, also made room to get a few beers on the way to the camp at night.
No flats, no problems. just fun!!