Shipments of tablets running Google’s Android mobile operating system are expected to pass Apple’s iPad this year as the overall tablet market grows, according to new data(Opens in a new window) from IDC.

The research firm on Tuesday predicted that Android devices will account for 48.8 percent of total tablet shipments in 2013, while Apple’s iPad will slip from 51 percent of the market in 2012 to 46 percent this year. IDC also increased its tablet market forecast for 2013 from 172.4 million units to 190.9 million, largely thanks to a “predicted surge in smaller, more affordable slate devices.

“One in every two tablets shipped this quarter was below 8 inches in screen size,” IDC research analyst Jitesh Ubrani said in a statement. “In terms of shipments, we expect smaller tablets to continue growing in 2013 and beyond. 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.”

By the end of 2017, tablet shipments are expected to reach upwards of 350 million units, IDC said.

In the long term, iOS and Android will give up some market share to Windows-based tablets, the firm said. But for now, tablets running Microsoft’s Windows 8 operating system are expected to account for just 2.8 percent of 2013 shipments, while Windows RT-powered devices will make up just 1.9 percent.

Microsoft is doing itself a disservice by trying to peddle two different tablet operating systems, IDC’s tablets research director, Tom Mainelli, said in a statement.

“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,” Mainelli said. “Such a focus could drive better share growth in the tablet category down the road.”

Meanwhile, growth in the tablet market is bad news for e-reading devices. IDC said it believes e-reader shipments peaked in 2011 at 26.4 million units before declining to 18.2 million units last year. The category is expected to “grow only modestly” this year and next, before beginning its “gradual and permanent” descent in 2015.