Sounds like the second file is hosed- try opening it in Wordpad, and seeing what the first line of text (read: gibberish) says. If you see the word DMG in there, then it's most likely a DMG file, and also that it's corrupted.
Try this with the first file as well (although it will take a very long time to load into Wordpad, so I'd suggest Hex Workshop) to see if it has DMG in the file as well. If it does, the file's definitely corrupt.
If it's a different format to the smaller file, then maybe you need to find a utility that will work with this file type.