Well, for one thing, seven figure book deals are extremely rare. Over the past few years, advances have gone down in general, but that has less to do with big advance books not paying out and more to do with the economy in general. Advances are tricky things. They are nice, but you have to keep in mind that the higher the advance is, the more it's going to take to earn out, and that puts a lot of pressure on the author and publishing house. Many books never earn out. It doesn't mean that the author won't get another book deal, but it may affect the size of his or her subsequent advances. There are a lot of factors that go into the size of the advance--market saturation, marketability of the title, how deep the pub house's pockets are, agent's negotiating skills, etc.
TWILIGHT and THG both drew more fans with the second book in the series than the first. I think that was about the time both became a "thing." I can recall only hearing the occasional mention of THG, but it suddenly everywhere when CATCHING FIRE came out.