What was the total amount of space burned on both discs? Select the VIDEO_TS folder, and check its properties.
Perhaps Nero burned up to 1gb if your original was less than 1gb?
When Nero does the copy process, do you drag anything into the Nero workspace, or do you just just tell Nero to make a copy? I'm wondering if Nero burned the new copies as "DATA DVD" instead of "DVD-VIDEO" (this might explain why the copies do not play in a dvd player).