As we can see in the graphs, for small l/d relations, which means that the nucleus is much smaller than the electron cloud, the energy range in function of tetta has a softer change.
In the case of l/d=0.6, the lowest energy is at-1.2, and the highest at-0.7, so there is a split of 0.5, meanwhile in the case of a l/d=0.05, the splitting is only of 0.012.
Then, we can conclude obviously that when the nucleus becomes smaller in relation with the electron cloud, the quadrupole term correction value gets also very small too. This is quite logic, because making the nucleus smaller means approaching to a point nucleus, a system in which the quadropole term has not any contribution.