I was reading quite a well-known paper titled 'Relativistic Effects in Structural Chemistry' by PEKKA PYYKKO.
In the paper, he describes how the electrons occupying $s$ and $p$ orbitals move at relativistic speeds and so orbit closer to the nucleus due to relativistic mass increase. He then states that due to orthogonality this causes the s orbitals in the higher principal quantum number $n$ orbitals to also contract. However, he states that for higher s shells because of 'interacting relativistic and shell structure effects, their contraction can in fact be even larger' than the 1s or lower s orbitals.
What I do not understand is what exactly the author means by interacting relativistic and shell structure effects, what are these affects specifically.