Birds can be larger than bats because of their body plan. Birds have long, strong feathered wings for lift, hollow air-filled bones that reduce weight, air sacs that aid breathing and reduce weight, and strong muscles for flapping.
Bats are mammals with wings made of skin and bone. They are relatively small, which helps reduce weight, and have agile bodies for maneuvering.
The combination of these adaptations allows birds to fly and reach larger sizes than bats.