Answer
The transition from $l=19$ to $l=18$ results in a larger calculated moment of inertia.
Work Step by Step
As explained on page 1411, the moment of inertia of a diatomic molecule depends on the distance between the centers of the 2 atoms. The molecule is both vibrating and rotating. When the diatomic molecule has more energy, and is spinning faster, its 2 atoms are vibrating with greater amplitude and spend more time farther apart, resulting in a larger moment of inertia.
