[allthingschm]

I read that as the principal quantum number increases, there are more nodes in the p orbitals which causes poor overlap between the outermost orbitals due to decreased electron density and hence result in weak bonding however it is also said that orbitals with similar size/shape interact more strongly. So in this case, I'm not too sure whether a 2p-3p orbital overlap is stronger than 3p-3p orbital overlap.

[mjc123]

Depends on the atom. For O, say, the 3p orbitals would be very large and diffuse, while for S, the 2p orbitals would be much smaller and in the core of the atom. So in P-O you would get 3p-2p interactions, whereas in P-S you would get 3p-3p interactions. Broadly speaking, valence orbitals interact with valence orbitals (surprise surprise).