Gpu's greatly benefit from parallelism. Cpu's not so much.
The "little" cores are great for low power use, ie; most of the time.
The "big" cores are great for when you play video that doesn't have hardware support. After adding a second core, faster clock speed would make a bigger difference.
Tablets and phones really don't have the screen real estate to support multi tasking, which is where multi core makes a difference.
That said, my little sgs1 is single core, and I last used a single core pc about 8 years ago, and the phone is a permanent reminder why I'd never want to go back to single core.
Rather than just big/little, variable clock rate & gated cores might be better...