I just downloaded the 1.5GB file and it appeared as 1.6GB for me as well in my storage. I haven't started the game yet as I purchased it here with this sale this week but...I'm concerned, fearing that once I do everything these DLC achievements won't unlock...
Anyways, I'm just confirming I have the same issue with having a 1.6GB file appear on my console like yourself. Hopefully someone comes in here with a fix or answer...why it lies and says it's 1.5 on the Marketplace I don't know.
Assassin's Creed III (Xbox 360), Splinter Cell HD (PS3)
Hannibal (Season 2), The Following (Season 2), Arrow (Season 1)