Typically such a bomb is a multi-level packing thing -- data's compressed with one packer (e.g. into a zip), then the resulting archive file is in turn packed (usually with a different packer), and so on several times.
We had a thread here a while back reporting avast and system crashes from trying to scan an apparently
small file (50 or 100K, if I remember) which would have eventually expanded, if disk space and memory were available, to a couple of hundred gigs.
So 4.5's new ability to at least try to detect such bombs is certainly a welcome addition.