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The iPhone 6 is outselling the larger iPhone 6 Plus by a 3-to-1 margin. It's partly due to supply constraints, and demand for Apple's latest is "surprisingly high" according to UBS. Apple Inc.

IPhone 6 sales are largely driven by the smaller model, a new study says. The iPhone 6 sold three times faster than the bigger iPhone 6 Plus, according to UBS analyst Steven Milunovich.

The iPhone 6 accounted for 68 percent of all iPhone sales in the first 30 days of its release in the U.S., with the 6 Plus taking 24 percent or less, Milunovich said in a recent note to clients. The new iPhones are more popular than last year’s models, according to the research. Ninety-one percent of total iPhone sales were the new phones in the weeks following their release, up from 84 percent in 2013.

IPhone 6 Plus sales are also constrained by supply shortages, the analyst said, so while the iPhone 6 should continue to outsell the larger model, it may do so by a smaller margin in the future.

Nearly one-in-five Samsung smartphone owners are planning to switch to Apple for their next purchase, Milunovich said, with most saying they will purchase the higher-priced iPhone 6 Plus. Demand for the iPhone in China is also “surprisingly strong,” he said.

Apple’s margins will also get a boost as customers pay more for 64 and 128 gigabyte models, according to the research. The company doubled its offerings for the mid- and higher-tier models.

UBS maintains a “buy” recommendation for Apple stock, and rose its 12-month price target to $125 from $115.