If the nucleus becomes smaller (i.e. l becomes smaller while d is kept constant) we can see that the energy range in which E0+E2 oscillates also becomes smaller (for l/d = 0.6 the range is approximately [-1.2;-0.7] whereas for l/d = 0.05 the range is [-0.808;-0.796]).
If we take a look at the expressions for E0 and E2, we can see that:
– the monopole energy E0 stays constant (because it only depends on d which remains constant) as can be seen on the graph (blue line)
– the quadrupole energy E2 becomes smaller because it depends on l^2 (which becomes much smaller as l goes down) and also on d^3 (which remains constant since d is constant)