A customer looks at a display for the Apple iPhone 5c and iPhone 5s in Paris, July 3, 2014. Balint Porneczi | Bloomberg | Getty Images

It's a big year for the iPhone, literally. At a choreographed , the entire tech world expects Apple to show off a device with a 4.7-inch screen, and possibly one with a 5.5-inch screen.

In a reversal of the stock's collapse two years ago, Apple has run up to all-time highs in anticipation of the bigger iPhones. It's a risky bet, because investors tend to cool on Apple in the fall after we actually see the long-rumored products.

This year, though, could be different.

Three factors suggest this could be an unusually strong year for iPhone sales. One, the upgrade-eligible pool is the largest it has been in years. Two, there's pent-up demand for a larger iPhone. And three, Apple's fortunes in China seem to have improved at just the right time.

First, it's important to consider the pool of consumers who are eligible to get a new iPhone.

In the past, that pool was limited to people whose two-year contracts were up, or whose carriers would allow them to upgrade early. Last year in the U.S., that pool turned out to be smaller than in years past because carriers cut back on allowing early upgrades in an effort to protect their profitability. (Wireless carriers lose money up front on subsidized phones, then make it back over the life of the contract.)

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But this year, carriers have figured out a work-around: New early-upgrade plans that essentially amount to phone lease agreements. In exchange for the option to upgrade yearly, customers agree to keep their phones in good working order and turn them in to the carrier when they're done. Customers also pay a higher monthly rate.

How popular are these early-upgrade plans? Very, according to a June report from research firm NPD. Upgrade plan enrollment more than quadrupled between September 2013 and the end of March, going from 7 to 31 percent. The reason is perfect for an iPhone launch: "Keeping up with the latest technology was cited by 84 percent of smartphone owners as the most influential reason why they signed up for an early upgrade plan," NPD wrote in June. "Money was next on the owner's mind, with 62 percent saying being able to finance the phone in monthly installments influenced their decision."