Resonance stabilization and size of ligand atoms
Yes, this is true. The $\pi$ overlap between a valence $\ce{p}$ orbital of a second row element and one of a higher row element is much smaller than the $\pi$ overlap between two second row elements. One reason is that the bond length is generally higher when higher row elements are involved (due to their size). Another reason is, that the $3 \ce{p}$ or $4 \ce{p}$ orbitals are much more diffuse than the $2 \ce{p}$ orbitals. If you want to read deeper into the topic have a look at:

[1] W. Kutzelnigg, Angew. Chem. Int. Ed. Engl., 23, 272 (1984).

[2] N. C. Norman, Polyhedron, 12, 2431 (1993).