Apple TV, imagined by Steve Jobs to be Apple's "fourth leg," now becomes its sixth

As noted by Horace Dediu of Asymco, just by itself Apple's newly isolated iTunes/Software/Services business "begins to look increasingly as a viable 'leg stool' upon which Apple rests."

That observation is an allusion to a comment made by Steve Jobs back in 2007, when he expressed, at the introduction of the iPhone and Apple TV, that, "we hope the iPhone is the third leg on our chair, and maybe one day Apple TV will be the fourth leg."

At the time, Jobs described Apple has having two legs, "the Mac business, which is a $10 billion business, and music, our iPod and iTunes business, which is $10 billion."

From two legs under $20 billion to six legs supporting $156 billion

Today, five years later, Apple's "stool" has more than four legs, but the company has shifted how it counts them. In 2012, the Mac grew into a $23.2 billion business, while the iPhone has grown into a $78.7 billion enterprise. The iPad, which didn't appear until 2010, is now a $30.9 billion "leg."

However, rather than counting iTunes along with iPod sales, Apple has separated them. That leaves iPods alone as a $5.6 billion business (which has rapidly been cannibalized by iPhones and iPads delivering iPod features via an app).

Separately, iTunes media, apps and other software and related licensing and services revenue has grown into a $12.9 billion business, alongside a $5.2 billion Accessories leg.

Categorized within "Accessories" are all of Apple's hardware peripherals and first and third party accessories (ranging from headphones to cases). This includes sales of Apple TV.

So rather than being Apple's "fourth leg" as Jobs imagined it might in 2007, Apple TV is filling out part of its sixth leg, narrowly behind the iPod but less than half the size of iTunes/Software/Services (which is now roughly half the size of both the iPad and Mac legs, which are both less than half the size of the iPhone).

Does Apple need new legs, or can it grow bigger ones?

A variety of observers are hoping that Apple will launch an entirely new product "leg" represented by TVs, watches, or other speculative new hardware products.

However, the blockbuster growth of Apple's existing six major businesses are already making once invisible products (like software and accessory sales) nearly as large in terms of revenue ($18 billion) as Apple's entire business was in 2006 ($19.32 billion).