Arcbomb is most likely a compressed zip/rar/7z file where the uncompressed file(s) size is much much higher (like an order or two magnitude higher) than the compressed one. This may happen when a file contain many repetitive texts such as forms and and templates. It also often happened if with older format of CD and DVD images when it only contains relatively small files and then got compressed.

This is almost always alerts modern OS because it may slows down the computer due to storage became full during file expand. However it is always wise to play safe.