"We have been churning out 140,000 iPhone 6 Plus and 400,000 iPhone 6 every day, the highest daily output ever, but the volume is still not enough to meet the preorders," a source familiar with the situation told Luk.
The iPhone 6 has a 4.7-inch display, and the 6 Plus has a 5.5-inch display. Apple's suppliers are having a harder time making the 5.5-inch display, which is why Foxconn is making fewer 5.5-inch phones.
Foxconn has 100 production lines cranking out iPhones "around the clock" in Zhengzhou, China, says Luk. There are over 200,000 people "dedicated to just making new iPhones and key components such as metal casings."
Foxconn is hiring more people to make the phones.
This is fairly standard for Apple. In the beginning, there's a crazy burst of demand and it's hard for its partners to keep up. Over time demand and supply balance out.