I don't think anything hollow is stronger than something solid, but it's much stronger for it's weight. Using a driveshaft as an example, if it was solid and say 3" in diameter, it would be strong but heavy. If it was solid and weighed as much as a hollow one, maybe it would be an inch or possibly less in diameter. That would be easier to bend than a 3" hollow one. A solid carbon fiber arrow should be stronger than a hollow one of the same diameter. A solid one weighing as little as a hollow one would be too skinny and unusable.