Why the iPhone 6 Could Be Apple's Biggest Success Ever

— -- The iPhone 6 and the iPhone 6 Plus are bigger than their predecessors in more ways than one.

The devices, which boast larger screens, are so popular since their unveiling last week that they have broken all iPhone pre-order sales records.

More than 4 million pre-orders for the phones were placed when they went on sale Friday, according to Apple.

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Demand has been so high that Apple said it has exceeded the pre-order supply, meaning that some customers won't receive their new phones until October.

If you haven't pre-ordered a new iPhone but are clamoring to have one by the end of the week, there's still a glimmer of hope if you're willing to wait hours or days in line. Apple said a limited supply of phones will be available in its stores for walk-in customers beginning Sept. 19 at 8 a.m. local time.

The iPhone 6 Plus was also in high demand from mobile carriers, who projected delivery dates stretching into October.

What makes the iPhone so hot, aside from having the latest Apple product, is the larger screen sizes.

The iPhone 6 is 4.7 inches, while the iPhone 6 Plus measures 5.5 inches diagonally, giving Samsung, which has dominated the giant phone market, some competition.

The iPhones are also packed with new features, including "reachability." With a double touch, the display can slide down so users can reach "anything" at the top of the phone, an ideal feature for multitasking.

Apple Pay, the company's digital wallet that is aiming to be a credit card killer, will also be supported on the new iPhones.