Is Grand Theft Auto IV - Episodes from Liberty City different from GTA IV?
Yes, it is different.
Episodes from Liberty City only contains the expansions Ballad of Gay Tony + Lost & Damned. It does not contain the storyline from the original GTA IV. I would assume the size is so large because it includes most everything from the original game. The Xbox 360 expansions both weigh in at 2GB+ by themselves. The expansions make use of most of the entire city and have all the same vehicles, guns plus more.