Clingy fabric, plus the big cowl neck, and the bottom-of-the-knee length... I suspect it all adds 20 pounds or so. But I think sweater dresses are generally unflattering, especially from the back, on most normal-sized people.
It's a little long but that's just my personal preference. I prefer just about the knee, and dresses are usually SHORTER on models than "normal" people because models are so tall! I've seen other sweater dresses I've liked better. I guess it depends on what style you're looking for.