Don't forget WAKE TURBULANCE.
A big jet can easily flip a small lighty flying up to 1000' or so below it's path and they would not have to be physically close either.
A 737 flies over the coast at 1,000 +, I minute later a PA28 flies under the previous path of the 737 and WAMMO.
They don't have to actually collide.
And I'd hate to be a lighty flying under a A380's path in YSSY.......yeesh.