Microsoft confirmed last week that it's working on a smaller version of its poorly selling Surface tablets, which are available in Windows 8 and Windows RT models. Much of Microsoft's mobile strategy has been to copy Apple in form, if not substance. Stuffing the already hard-to-use Surface Pro into a smaller screen is a recipe for greater failure.

Apple did not invent the 7-inch tablet. The first ones did follow the original iPad, as companies like Samsung, Dell, and Asus tried to join the tablet revolution after the fact by taking the smartphone version of Android and running it enlarged on a larger device that couldn't make calls. These flopped big-time, and the iPad dominated the tablet space for three years. Yes, in November 2011, Amazon.com released the Kindle Fire, an anemic tablet designed for consuming Amazon content; it got a lot of interest at launch, but that petered off quickly.

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But in spring 2012, Google's Nexus 7 launched a wave of small tablets that found a real audience. Amazon.com followed with a revamped Kindle Fire, Barnes & Noble with the Nook, and Apple with the iPad Mini. Like the Kindle Fire and Nook HD, the Nexus 7 was aimed at media consumption, but unlike those rivals, it ran the full Android tablet OS under the media-focused home screen. The iPad Mini also runs the full iOS and doesn't bother trying to focus on media consumption alone. Samsung likewise has a small tablet line running standard Android: the Galaxy Tab 2 7 released a year ago and the Galaxy Note 8 released this year.

You can see why Microsoft wants one, too. But there's a key difference between the successful 7-inch tablets and how Microsoft is likely to deliver its own. (Microsoft has revealed no details of its plans, only confirmed it has plans.)

Apple's approach to the iPad Mini was to have it run all iPad apps unmodified: The iPad Mini's 7.9-inch screen is 85 percent that of the full-size iPad's 9.7-inch screen, so most apps are readable and tappable as is. The pixel ratio is also the same as for the iPad 2: 1,024 by 768. Likewise, Google's Nexus 7 and Samsung's Galaxy Tab 8 run Android in its standard 1,280-by-800-pixel resolution. On a 7-inch device, that's a bit pinched, but acceptable on an 8-inch display. In other words, when you buy one of these small tablets, you get a much lighter, smaller device at the price of perhaps needing reading glasses from time to time.

Amazon's approach was to not try to be a general-purpose tablet but a media device, for watching videos, reading books, and playing games. Yes, there's a browser and an app library, but they're not why you get a Kindle Fire.

What about Microsoft? On tablets, Windows 8 requires a minimum screen dimension of 1,366 by 768 pixels. Although Metro apps may be comfortable to see and manipulate at that resolution on a smaller tablet, the Windows 7 Desktop will not -- it's already too hard to read and manipulate on the existing 10-inch Surface Pro. Also hard to use at that resolution is Office 2013 and its not-Desktop, not-Metro UI on both the Surface Pro and Surface RT.