iPhone 6 Plus is a rocket fuel for phablet sales in the U.S.

Sales of outsized smartphones, or phablets, have almost quadrupled in the United States over the past year — thanks mostly to Apple's new larger iPhones.

That's according to numbers from market research firm Kantar Worldpanel ComTech, which found that phablet sales have spiked since the release of the iPhone 6 Plus.

Phablets, typically defined as handsets with 5-in. displays and larger, made up 21% of all smartphone sales in the United States during the first quarter of 2015, says Kantar.

This represents a 15% increase from the first quarter of 2014, when phablets made up just 6% of U.S smartphone sales.

Fueling this growth was Apple's 5.5-in. iPhone 6 Plus, which accounted for 44% of the phablet market, the report says. Together, Apple's flagship smartphones, the iPhone 6 and 6 Plus, now account for 18% of all iPhones in the U.S.

The numbers highlight just how important screen size has become — "screen size was cited as the main reason for buying a particular phone by both iOS and Android buyers at 43% and 47%, respectively," the report says.

Before Apple launched the iPhone 6 and 6 Plus last fall, earlier reports suggestedthe company needed larger displays in order to stay competitive with rivals like Samsung.

Exactly why users are drawn to larger displays is unclear, but there are likely several factors at work. Adobe principal analyst Tamara Gaffney speculated last fall that the widespread availability of free wifi is driving more users to stream video while on the go — and other activities that are more satisfying on larger screens.

An earlier report from Kantar found Apple's larger phones were also causing changes in the European smartphone market, with more Android users switching to iOS during the first quarter of 2015.

Across Europe, Android's marketshare decreased by 3.1%, the report found, though the operating system still dominated at 68.4%. The report points to Europe's "Big 5" — Great Britain, Germany, France, Italy, and Spain — as having the biggest increase in iOS users where Apple's marketshare is 20.3%.

For Windows Phone, however, the numbers are much less promising, despite its own flagship phablets in the Lumia line. Both reports note that Microsoft's operating system has "grown little" in the U.S and Europe. That's with the exception of France, where Windows Phone claims just over 14% of the market.