Worldwide tablet shipments are expected to reach 221.3m units in 2013, down slightly from a previous forecast of 227.4m but still 53.5% above 2012 levels, according to the latest forecast from the International Data Corporation (IDC) Worldwide Quarterly Tablet Tracker.

Shipment growth is forecast to slow to 22.2%, year on year, in 2014 to a total of 270.5m units. By 2017, annual market growth will slow to single-digit percentages and shipments will peak at 386.3m units, down from the previous forecast of 407m units.

IDC highlighted the importance of the mix of small versus large tablets in projecting the slowing growth. The market has trended toward small tablets markedly over the past two years, but the rise of large phones could well push consumers back toward larger tablets as the difference between a 6-inch smartphone and a 7-inch tablet is not significant enough to warrant purchasing both, IDC argues. Apple's launch of the iPad Air, a much thinner and lighter version of its 9.7-inch product, could herald another market transition back toward larger screens, if consumers are willing to pay the higher costs associated with bigger displays.

"In some markets consumers are already making the choice to buy a large smartphone rather than buying a small tablet, and as a result we've lowered our long-term forecast," said Tom Mainelli, research director, Tablets.

"Meanwhile, in mature markets like the US where tablets have been shipping in large volumes since 2010 and are already well established, we're less concerned about big phones cannibalising shipments and more worried about market saturation."

A transition toward larger tablets could be a positive development for Windows tablets, which generally benefit from a larger screen area. Even so, Windows-based tablets are not expected to steal share from tablets running iOS and Android until the latter part of the forecast.

"For months, Microsoft and Intel have been promising more affordable Windows tablets and two-in-one devices," said Jitesh Ubrani, research analyst, Worldwide Tablet Tracker.

"This holiday season, we expect a huge push for these devices as both companies flex their marketing muscles; however we still don't expect them to gain much traction. We're already halfway through the holiday quarter, and though there have been some relatively high-profile launches from the likes of Dell, HP, and Lenovo, we've yet to see widespread availability of these devices, making it difficult for Windows to gain share during this crucial period."