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An oversize bag is dangerous because it invites you to carry tons of stuff, but it isn't intrinsically worse than a small one. Shape and dimensions matter less than how the bag meshes with your body, the doctor says. If it's on your shoulder, it should automatically find the sweet spot between your side and arm. If it's over your forearm, it should be right at your waist (which means women with long torsos or a lot of height can afford a deeper bag).