Hungry Horse,
Bore size has nothing to do with spread, unless you are talking just a few feet.
it's general boring or choke that tightens up pattern.Yes, some bore always shoot tighter than others, but this goes for all bores. there is no general rule that says small bores shoot tighter.
If interested, a copy of GT Garwood's book "Shooting facts and and fancies" will clear it up.
His works are excellent, and as a civil engineer was not the type to go off hearsay.
All the best,
Richard.