How do these molecular orbitals differ from those obtained from linear combinations of the 2py atomic orbitals? (the 2py orbitals are also oriented perpendicular not only to the bonding axis but also to the 2pz orbitals.) ?

Size of the AO s (for best overlap, orbital should be same size) Symmetry of the orbital (2px cannot combine with 2py or 2pz since both are orthogonal; also s orbit can only overlap with a p orbital end-on since sideways overlap lead to no overall gain in energy)