I only have 2 'installed' BluRay games on my PS3, and that's VF5 and Ninja Gaiden. Upon downloading the MGS4 trailer this morning, I noticed I had 39 GBs of free space left. I was surprised that so much of my 60 GB harddrive space had been eaten by 2 games.
Not to say the 2 games alone took up 21 GBs. Boring details in spoilers. Skip past for the result.
This is mostly anecdotal evidence, as I don't have my PS3 here to check the real numbers.
I only own 2 other games, Resistance and
Gundam Musou, and those read off the disc. I have 4 demos installed (Genji, Armored Core, Ninja Gaiden, Rub a dub) and one HD video of around 250 megs. Above all that, I have 3 full PS2 cards installed on my drive. Assuming the demos are 1GB each (they are not), all of the other data that I loaded myself should only be 4.5 GB. Add to that around 2 GBs of system data and updates, and that's 6.5 GBs of data used for stuff that is not installed games.
60GBs (PS3 max storage) minus 6.5 GBs (other data) = 53.5 GBs remaining space.
So if my current 'remaining disk space' after installing 2 games is 39 GBs:
53.5 GBs - 39 GBs = 14.5 GBs.
Those two installed games together equal at least 14.5 GBs.