Problem starting up rpcs3.exe

PS3 ISOs may be encrypted, when you create an ISO you may need to decrypt it first to properly unpack it. So, even if the game occupies 6GBs, if the contents are encrypted then it won't work.

You can extract firmware modules from your ps3 or from a software update file called PS3UPDAT.PUP using a PUP extractor. We don't have a guide on that yet, though, but you should be able to find many guides on YouTube for that purpose.