Size on disk isn't always a good indicator of digital size, as there's less pressure to compress data. Especially on the PS3.
That bring said, the usual culprit of modern game size is HD video. Video captured at 720p is already large. 1080p is massive. Games these days, when using pre-rendered video, opt to use those resolutions as anything less on modern displays tends to look like bollocks.
Audio also chews up a lot of space. Lots of dialogue will add up, and something like the GTA radio stations speak for themselves.
Coupled with the growing size of high quality assets, like textures, it's no surprise games are huge. These days PC games are probably on average ~10GB, a little more or less depending on the game, with some extreme examples like Max Payne 3 and Shogun 2 passing 20GB.
You've also got to factor in DLC, which is quite large these days. Bigger stuff like the various Battlefield 3 packs really add up.
If you're planing to do a lot of full game digital purchases, neither Wii U unit will cut it. Better invest in a portable HDD.