Theres 20 GB models out there it was around 08 they launched a 20 and 80 GB versions these were the last of the models supporting full backwards compatibility they got phased out a few months later. As for the install size its not really an issue for me I've got 35 GB free and can easil delete another 14 GB if I ever need it, I don't think skyrim will have that big of an install on the ps3 most of them are fairly small.
If you read the RAGE preview they say its got a 22GB to install which is already over my hard drive space and I think Skyrim will be just as big if not bigger. So I don't want to waste £150 on a 250GB hard drive and a data transfer cable. And from the reviews I've seen on these cables they say when they connect it to their PC they got Trojans and other viruses.
So when Skyrim comes out, after an hour of playing I notice loading takes much longer (like Fallout: New Vegas) So I decide to install the game, which takes hours and takes up probably quite a lot of hard drive space and I believe most gamers have taken up at least 3/4 of space for other games meaning they have to delete other stuff, so when they want something new they might uninstall the game but when they come back to it later they have to download it again wasting more time.
But other games like Mass Effect2 and L.A. Noire have multiple disks, which I think is much more helpful than huge downloads as those games I only remember having to change the disk like a few times which is even quicker than loading screens if your disks are already out ready to swap.
So in the end isn't having 2 disks more efficient and cheaper than huge downloads, unless its a psychological way of making us waste more money by buying more equipment to make gaming more speedy and enjoyable, possibly the reason why DLC is coming to Xbox first maybe because Todd Howard deliberately made it on one disk so Microsoft get more money via people spending money on hard drives and other equipment so Microsoft bribed Beth so they gave Xbox DLC first this way Xbox users get happy and buy more stuff, its like a cycle.