Shipments of Apple's iPads will fall behind the tablets running Google's Android platform for the first time this year as smaller-sized devices catch on with more consumers, research firm International Data Corp (IDC) said on Tuesday.

A predicted surge of smaller, lower-priced devices in the tablet market has led IDC to increase its 2013 forecast for the worldwide tablet market to 190.9 million, up from its previous forecast of 172.4 million units.

Increases in tablet shipments have been made throughout the forecast period with an average increase of 11% between 2013 and 2016. IDC's latest forecast update estimates tablet shipments to be upwards of 350 million by the end of 2017.

"One in every two tablets shipped this quarter was below 8 inches in screen size. And in terms of shipments, we expect smaller tablets to continue growing in 2013 and beyond" said Jitesh Ubrani, Research Analyst for IDC's Tablet Tracker. "Vendors are moving quickly to compete in this space as consumers realize that these small devices are often more ideal than larger tablets for their daily consumption habits."

Android-based tablets expanded their share of the market notably in 2012, and IDC expects that trend to continue in 2013. Android's share of the market is forecast to reach a peak of 48.8% in 2013 compared to 41.5% in IDC's previous forecast. Android's gains come at the expense of Apple's iOS, which is expected to slip from 51% of the market in 2012 to 46% in 2013. Longer term, both iOS and Android will eventually relinquish some market share to Windows-based tablets, with Windows 8 predicted to grow from 1% of the market in 2012 to 7.4% in 2017. IDC expects Windows RT growth to remain below 3% during the forecast period.

Tablet Operating Systems, Forecast Market Share and CAGR 2012-2017

Source: IDC Worldwide Quarterly Tablet Tracker, March 2013

Windows shipments include Windows 8 and Windows 7 tablets / Shipments include shipments to distribution channels or end users. OEM sales are counted under the vendor/brand under which they are sold.

"Microsoft's decision to push two different tablet operating systems, Windows 8 and Windows RT, has yielded poor results in the market so far," said Tom Mainelli, Research Director, Tablets. "Consumers aren't buying Windows RT's value proposition, and long term we think Microsoft and its partners would be better served by focusing their attention on improving Windows 8. Such a focus could drive better share growth in the tablet category down the road."

While IDC continues to revise its tablet forecast upward, the firm had done the opposite with the eReader forecast. The growth of low-cost tablets is clearly damaging the prospects of the single-use eReader, and IDC reduced its forecast for the category by an average of 14% between 2013 and 2016. IDC believes eReader shipments peaked in 2011 at 26.4 million units. After declining to 18.2 million units in 2012, the category is expected to grow only modestly in 2013 and 2014, before it begins a gradual and permanent decline beginning in 2015.