Having generated revenue of $13.5 billion last year, Apple’s iTunes is now the company’s fourth largest business according to Asymco analyst Horace Dediu.

iTunes, up from $10.2 billion in 2011 to $13.5 billion in 2012, has averaged a growth of 30 percent over the course of the past two years.

Dediu suspects that if trends continue, as Mac and iPod sales decline, iTunes could very well become the company’s third-largest business sometime this year. The Apple Accessories line is already larger than the Mac, but it’s important to keep in mind that Apple’s iTunes and accessories are dependent upon other hardware sales.

“The size of these ancillaries are beginning to be substantial and Apple’s ecosystem may begin to be valued more than just as an incentive to buy the devices. Indeed, if seen in isolation, iTunes plus Accessories combined is a bigger business in terms of revenues than any of the other phone vendors except Samsung,” Dediu said.

ITunes alone is now larger than Microsoft’s Xbox and Windows phone divisions, combined.

Here’s something else to keep in mind, Apple’s iTunes revenues for 2012 don’t include $7 billion paid to app developers. Dediu clarified that Apple’s “iTunes/Software/Services” counts wholesale revenues of music sales, but onlu the company’s 30 percent cut of apps within iTunes revenue.

Apple recently confirmed that iTunes users had surpassed more than 25 billion songs since the iTunes Store inception in April of 2003. Apple’s senior vice president of Internet Software and Services, Eddy Cue, said in a press release, “Averaging over 15,000 songs downloaded per minute, the iTunes Store connects music fans with their favorite artists, including global sensations like Adele and Coldplay and new artists like The Lumineers, on a scale we never imagined possible.“

[Image via iTunes screenshot]