Heya,
I encountered weird behaviour with a large (4gig) archive compressed with os x (10.5.1) archive utility. It just wouldn't extract right with the unarchiver and always ended up with some 500 megabyte file instead of the 4 gigabyte original. Archive utility on the other hand could extract it a-ok. First I thought it was an issue with the archive and compressed the file again a few times before I figured out I could try archive utility.