How does the length of the bridge of the resulting bicycle change the rate of the Diels Alder reaction (examples below)? I can't seem to think of how one would be more electron donating than the other to result in a more electron rich diene.
Assuming the claimed difference is reaction rates is real (see the comments), we may want to consider the distance between the ends of the conjugated diene chain. You need these ends to overlap with the pi orbitals of the other alkene, and making the diene end-to-end distance closer would improve the overlap thus facilitating the reaction. Thus the pentagonal ring, by creating a closer distance than the hexagonal one, is expected to lead to a faster reaction rate.