Answer
The wings of a hummingbird are smaller/shorter than those of the eagle relative to its own length. These short wings have small rotational inertia, allowing them to flap rapidly.
If a hummingbird were scaled up to eagle-size, its relatively stubby wings would be too small for it to soar. The surface area grows less rapidly than the mass.
An eagle's long wings, suitable for soaring, would be much too difficult to flap rapidly because of their high rotational inertia.
