I'm quite way behind the loop when it comes to PS3 development, but last I heard (Like a really REALLY long time ago, so please correct me if I'm wrong), most games don't fully utilize the BR disc completely and instead create duplicate sections of data to reduce data read times in order to compensate for the low disc read speed of the drive.

So, if that's the case, there's a high chance the total download won't be as large as a full BR disc and since you can take advantage of compression and whatnot, could go even smaller.