About high powered ammo and rifles?

i get confused.. which ammo is bigger and powerful? .338 or the 7.62? i thought the .338 is small coz well.. it's .338! and i got more confused by .50 cal!! i thought 7.62 is the biggest. by the way if the barrett is the most powerfull sniper rifle out there what's the second most powerful? AWP? i heard it uses .338, .308 and can use 7.62x51 also??