Remember that tablet boom? A new report from industry research firm IDC forecasts a lower growth rate for tablets this year.

That forecast Relevant Products/Services, released Thursday, doesn't project an actual decline in the sales of tablets. In fact, it still expects a 12.1 percent year-over-year growth rate -- but that's a sizable drop from the phenomenal year-over-year growth rate of 51.8 percent in 2013.

Tom Mainelli, program vice president of Devices and Displays at IDC, said in statement that the relative slowdown is due to two factors.

"First," he said, "consumers are keeping their tablets, especially higher-cost models from major vendors, far longer than originally anticipated." When consumers do buy a new one, he said, the existing tablet often gets passed "off to another member of the family," cutting down on replacement sales.

Rise of Phablets

Additionally, Mainelli pointed to the rise of phablets, which are the name given to smartphones with screens that are 5.5 inches to 7 inches. These devices, he said, "are causing many people to second-guess tablet purchases as the larger screens on these phones are often adequate for tasks once reserved for tablets."

Phablets, in contrast to tablets, are showing an increase in their growth rate. The share of smartphone shipments has more than doubled since the first quarter of last year, when it was 4.3 percent, to 10.5 percent in the first quarter of this year. IDC said it expected the market to shift back to larger-screen tablets, like the new, Microsoft 2-in-1, 12-inch Surface Pro 3.

The research firm said that the shift back would help to double the number of Microsoft-based devices in this category by 2018, and will give a boost to vendors, since tablets are, on average, 50 percent higher than phablets in price.

IDC projects that devices with screen sizes of 11 inches and larger will grow from about 0.9 percent last year to a forecast share of 1.9 percent next year, and 6.6 percent by 2018.

Models from 8 to 11 inches, which represented about 44 percent of tablets in 2013, are projected to become about 47 percent of sales this year and nearly 49 percent by 2018. And devices from 7 to 8 inches, accounting for 55 percent last year, will drop to about 45 percent by 2018.

Smartphone Report

In a separate report released by IDC Wednesday, its Worldwide Quarterly Mobile Phone Tracker predicted that worldwide smartphone shipments would reach 1.2 billion units by 2014, representing a 23 percent jump from the 1 billion shipped last year. By 2018, global smartphone sales are expected to reach 1.8 billion units.

Ramon Llamas, research manager with IDC's Mobile Phone team, said in a statement that the largest growth would happen in the key emerging markets: India, Indonesia and Russia, markets where shipments would more than double from now to 2018.

The average selling price of smartphones will hit $314 this year, a drop from $335 last year. By 2018, it is expected to be about $267.