Why can flying birds be so much larger then flying bats?

There are a few reasons. First and foremost a bird is lighter than a bat of the same size. Birds have hollow bones, which makes their skeletons very lightweight. Second, flying requires a lot of energy, and the bigger the animal the more energy it takes. Birds have a complex respiratory system that is more efficient than that of any mammal to meet these energy needs. Finally, the largest birds are mostly gliders. The largest flying birds are primarily gliders. The airfoil shape of a bird wing is better suited for long glides than the membrane wings of a bat.