They're talking about how 'natural' a headphone sounds, which includes not only spatial effects but is dependent on the acoustic impedance of the headphone, and it's this acoustic impedance that Oratory's saying can be affected by driver size. I believe these acoustic impedance effects only really come into play for over-ear headphones below ~500 Hz, below the frequencies which I'd think contribute to most spatial effects, but even so, yes strictly speaking every physical parameter of a headohone that affects its frequency response can affect its spatial effects, including driver size, but that's very different from the audiophile trope of 'bigger driver => bigger soundstage'. There's no such simple relationship there.