This is a massive pet hate of mine.. i park miles from other cars and I always find a big 4x4 or people carrier next to it when i get back.
I used to think parking next to a nice car will save you.. but this is often not the case - as they are often just as careless.
imo i would have made a snide remark and left it at that... if there was considerable damage - so cracked paint i would take a pic, and go up to them (regardless of thier size) and get their insurance details... or let my insurance deal with it.