It depends where you are in the PZT phase diagram. But, in general, La is less diffusive than Lead Oxide, so, grain growth is reduced, as the grain growth is controlled in a large part by equalizing the chemical potential via diffusion.

Thanks Mr. Carlos Paz de Araujo 

This composition is near to the morphotropic phase boundary. Can you please elaborate your answer in detail or provide some references to understand this phenomenon.

I agree with Carlos, because of lesser diffusion of La3+ ion in PZT we get smaller grains. By lanthanum substitution we get round shaped grains. Transition temperature decreases as La decreases tetragonality of he PZT system. 

The reduction in grain size with the addition of La could be attributed to the impurity drag mechanism in solid solution in which the La ions gradient is present at the grain boundaries. Another reason for the decrease in grain size is that the addition of La might have restricted the grain growth by resisting the mobility of the grain boundary.