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?
as linus torvalds puts it: big numbers confuse me, so the next kernel version will be 7.0
No, the major number has no special significance to OpenBSD. 8.0 is just what comes after 7.9.