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.
I thought I had something meaningful to put here but I'm at a loss so...booty-butt-cheeks. Oh and thanks to Gackt for the Signature. Because OP Man and Japanese Iron-Man are too beastly.