It's now eight months since Apple (NASDAQ:AAPL) launched the iPhone 5s and the device is still the best-selling smartphone in the world. Even after the newer, and much bigger, Samsung (NASDAQOTH:SSNLF) Galaxy S5 debuted, the 5s continues to outpace other smartphones, according to new data from Counterpoint.

Here's why this matters:

Analysts believe there's pent-up demand for a larger iPhone 6. Brian Marshall, an analyst at ISI Group, thinks current iPhone owners could upgrade to the larger iPhone 6 at a 14% upgrade rate, more than the average 9% each quarter.

It's no secret Apple knows consumers want a larger iPhone. Smartphones currently on the market skew to sizes much larger than Apple's current 4-inch display size.

So if there's pent-up demand (i.e. people waiting to buy a new iPhone until a larger one debuts) and the iPhone 5s is still the No. 1 selling smartphone right now then we could see some huge sales numbers for Apple's upcoming device launch. Morgan Stanley analyst Katy Huberty thinks a larger iPhone could bring an additional 15 million more iPhones with a larger form factor and Apple suppliers expect a 20% increase in iPhone sales because of the larger displays.

Obviously we don't know exactly how many iPhones Apple will sell, but with iPhone 6 building on the success of the 5s and coming in at least one new size (if not two), then it's realistic to think the new device could easily be more successful than its predecessor.

An encore performance
While tapping into pent-up demand for a larger iPhone is great for Apple, investors should remember that once the iPhone 6 debuts, the 5s will continue to bring in significant revenue for the company.

Charlie Wolf of Needham & Company projected the iPhone 4s sold 10 million units and accounted for 25% of the iPhone revenue for the March quarter.

At the end of May the iPhone 4s took the No. 6 spot on the best-selling smartphone list, even though it's three years old, has a screen size of just 3.5-inches, and a slower network connection.

Now consider the best-selling, 4-inch, LTE-enabled 5s with Touch ID, will sell for a cheaper price than ever before. For consumers looking to upgrade, but can't afford a new iPhone 6, the 5s is a tempting option.

Why best-selling devices matter
Apple derived 57% of its total revenue from iPhone sales in Q2 2014, so the company clearly wants its devices to stay at the top of the list. When the iPhone 6 launches this fall and the 5s loses its flagship status, investors would be wise to keep track of the older devices' sales. If analysts are correct about their pent-up demand estimates, Apple could take two of the top-selling smartphone spots.

I'm bullish on the iPhone 6, in either one or two new sizes, and it stands to be a huge success for Apple. The 5s already proved its desirability and there are only a handful of major improvements in the device over the iPhones 5, including Touch ID, 64-bit processing and the M7 processor. The iPhone 6 will be a larger, more powerful, and probably slimmer device than all of the iPhones before it. If there's ever going to be a phone that can outperform the best-selling iPhone 5s, it's the bigger and much-anticipated iPhone 6.