Why do birds have a wingspan roughly twice that of their body length while bats have a wingspan roughly five times that of their body length?

Birds, Unlike mammals, have hollow bones. This makes their skeletal structures light enough to allow prolonged flight. Bats, on the other hand, have solid, marrow-filled bones. They require the extra force generated by a broader wingspan to lift their bodies and propel themselves through the air.

Simple answer: Bats have heavier bodies by size.