As someone that emulates, I can confirm that you can see the filesizes slowly creep upwards with each generation. The Largest I've seen from a PS2 game is about 4-5 GB and that's with games like GTA: Vice City and Tony Hawk's Underground. Then you get to the PS3 and all of a sudden there's these 15-30 GB titans like Metal Gear Solid 4, Heavy Rain and The Last of Us.

It's a safe assumption to say that if the devs can realistically acquire a certain amount of space for a game, they'll move to fill as much of it as possible. This wasn't a problem with previous generations because they were stuck with physical media, if they wanted you to get their game, they had to fit it on a disc. But with digital downloads they don't have to worry about their game being too big for the disc because the only constraint is your Hard Drive and that's on you, not them.

Hopefully, before too long we get some sort of major advancement in terms of storage space and huge capacity stuff becomes really cheap all of a sudden, but then there's the possibility that they'll just see that as a green light to make their games even larger.

TL;DR: Devs don't deserve the unrestrained freedom digital downloads have given them.

I find pathetic how most of video games nowadays have so much gigabytes because of the GODDAMN GRAPHICS.