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On Monday, Apple (NASDAQ:AAPL) reported its second-quarter earnings. And in what's becoming quite common for Cupertino, it surpassed analyst expectations. On an earnings per share basis, the company reported $2.33, up 40% from last-year's report (split-adjusted) where The Street expected $2.16. As far as top-line goes, the company reported revenue of $58.01 billion, up 27% from last year's report, versus the analyst estimates of $56.03 billion.

The EPS figure appears to be aided both by Apple's massive buyback program and by margin expansion (more on that later). During the conference call CFO Luca Maestri noted the company spent $7 billion to repurchase 56.4 million during the quarter. However, if this report had one big theme it would be the iPhone's massive growth. Here's a visual representation of Apple's last eight quarters by total revenue, iPhone revenue as a percentage of total revenue, and year-over-year iPhone revenue growth.

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Source: Apple's 10Qs. Revenue in millions, other figures in percentages.

The iPhone 6 and iPhone 6 Plus effect
As previously mentioned, Apple increased total revenue 27% on a year-over-year basis, reporting $58.1 billion versus last year's $45.6 billion but that doesn't tell the entire story. In each named product category outside of the iPhone, Apple did not attain that growth rate on a revenue basis. The Mac line and services lines grew 2% and 9%, respectively, while the iPad and other products categories fell 29% and 10%, respectively. Fortunately for Apple investors, the newest iterations of the iPhone came to the rescue in a big way.

As the chart shows, Apple's largest category contributed heavily to the top line this year. By growing nearly 55% on a year-over-year basis, the iPhone accounted for nearly 70% of Apple's revenue haul this quarter. Apple benefited by both a higher average-selling-price, or ASP, of $659 versus last year's $596 -- an increase of nearly 11% year over year as investors snapped up the more-expensive, larger form factor -- and also by increasing units sold by 40% year over year.

Upsides and downsides of single-product reliance
For Apple investors, this is a double-edged sword. On one hand, it is widely assumed that Apple's iPhone is its largest-margin product. And as that product continues to dominate Apple's product mix, investors will continue to reap the benefits of large margins -- for example, this quarter's gross margin percentage came in at a higher-than-expected 40.8%. Those larger margins tumble down the income statement to shareholder earnings as higher margins and share buybacks produced a 40% year-over-year EPS increase off of a 27% revenue increase.

On the other hand, however, this can result in a riskier investment. While Apple had an amazing quarter, and should be commended for its operational performance, this was on the basis of one product. In the event this product is challenged by new entrants, existing competition, or the bargaining power of suppliers or customers, the company could find it hard to make up for the loss of iPhone revenue from other products.

That said, it doesn't appear like that is going to happen anytime soon. Right now the company is coming online in China in a major way. This quarter was the first quarter in Apple's history in which it sold more phones in China than in the United States, Bloomberg reports, as Apple was aided by Chinese New Year. Apple's doubling down in China by nearly doubling its store count in Greater China. It appears Apple is just getting started in the world's largest country.

Jamal Carnette owns shares of Apple. The Motley Fool recommends Apple. The Motley Fool owns shares of Apple. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.