Does the version number going from v7.X->v8.0 mean that this is a larger update than v7.8->v7.9, or does the numbering just automatically roll over to the next major version when it reaches vX.9?
No, the major number has no special significance to OpenBSD. 8.0 is just what comes after 7.9.
as linus torvalds puts it: big numbers confuse me, so the next kernel version will be 7.0
He ran out of fingers and toes to count on. (Wait, what comes after 19.19, or 20.19, 20.20, then what?)
By that point medicine will be sufficiently advanced that we can graft new fingers on him, obviously. That'll also make him more productive, given he's an emacs user.