Microsoft Surface sales have been rather modest so far, with recent estimates putting the number at 1.5 million since the device launched in October 2012 -- 400,000 of them for the more expensive Surface Pro unit which launched in February. But the company is already planning an addition to the lineup, according to the Wall Street Journal, in the form of a 7-inch model that's going into mass production later this year.

The paper's unnamed sources claim that the company had no plans to add a smaller model initially, but felt it had to respond to the growing popularity of the form factor -- with Apple's iPad mini, Google's Nexus 7 and Amazon's Kindle Fire as prime examples. Market research firm IDC recently reported the steepest drop in shipments for the PC market in the last 20 years, citing the rise of tablets as one of the reasons for the decline, and noting that more than half of all tablets that shipped in the fourth quarter of 2012 were of the sub-8-inch variety.

This isn't the first time we've heard about an upcoming 7-inch tablet form Microsoft. Back in November The Verge reported on a rumor claiming that the company was readying a 7-inch gaming tablet called Xbox Surface that will include a custom ARM processor and custom Windows kernel optimized for gaming tasks.

The Journal's report doesn't mention gaming so it's unclear if this is the same device they're talking about, or if  a gaming-centric tablet to launch around the same time as the next Xbox is even in the pipeline anymore.

Other hints pointing to smaller Windows 8 tablets include a recent Windows 8 policy change allowing devices featuring lower-res screens, and rumored changes as part of Microsoft's upcoming 'Blue' update that will bring new snap view options that would supposedly improve side-by-side app performance on 7- and 8-inch devices.