Newborn diapers too big.

My little guy was born at 7lb and lost a few oz. The newborn size diapers seem really big on him. I was considering buying him a pack of preemies until he gained a little more weight, but they say they are up to 5lb. Is this really acurate or only a guess? Are the preemie diapers THAT much smaller than the newborn ones? Because he's got a lot of excess room in the newborn. I don't want to buy a pack and not be able to use them...