Since we are talking lenses, let’s liken it to the focal point, or “burn point” of a magnifying glass: (As if you didn’t already know, I’ll do a silly little pretend lecture) If you hold up a magnifying glass so that its lens axis optimally points at the sun, the light that comes into the lens across its entire area will refract and converge, on the other side of the lens, on a point a certain distance from it, given by the lens profile, and the relative refractive indices of the lens and the surrounding media (let’s say glass and air :7).
Put a piece of paper in this spot in 3D space, where all the light that enters the entire area of the lens is concentrated in a tiny point, and you can set fire to it.
That point could be called a sweet spot for setting fire to a piece of paper using that magnifying glass; Just a tiny bit away from it in any of the three spatial dimensions (although less abrupt in the lens-relative Z axis), there will be no concentrated heat - you are out of the sweet spot. (…although the lens is never perfect enough that the spot is infintessimally small - there will of course be deviations that limit how small you can project the spot onto the paper (EDIT2: …not to mention that the sun “rays” are not parallel - the sun is quite big in the sky, and you get light from both opposite sides. :7)).
Similarly you need to have your eyeball properly aligned with the lenses in a VR headset, for best optical conditions - both positionally and rotationally. There can be several aspects to this, and a lens may not necessarily have the sweet spot for them all in the same place (e.g. are the ridges between the segments of a given fresnel lens parallel with one another, or do they converge somewhere, so that you see them edge-on?), but let’s not go there…
When you are perfectly aligned, you should achieve the best the HMD can offer in terms of, amongst other things, that other definition, from the previous post, which is probably where the confusion stems from: Mixing up cause and effect, exacerbated by the not unreasonable argument that the little area in focus, in the midst of progressively worse blur toward the edges, could indeed be described as a “spot”.
A “large” sweet spot would be one that has a lot of tolerance, so your eye could move a little bit away from the singular point in space, relative to the lens without the view through it becoming all too distorted and out of focus, making it more a volume than either point, line, or disc, that you can move around within, and still keep the contents of your bowels :P, but that volume remains centred around the one spot in space.
Now, an HMD has to somehow accommodate some movement away from the perfect singular point, of course, with tolerances, or tunings, since looking around moves the pupil of one’s eye across the space in front of the lens (EDIT: fixed lens, that is – if we had that strong a contact lens, the matter would be radically different).
(…and here I can make an example of the matter of different aspects I didn’t want to go into earlier: The Oculus Rift CV1 could be said to have a large sweet spot, because you can wear it pretty wrong, and still get a sharp-ish view, but you may still experience other abberations caused by being off, such as the view warping, and colours separating.