Using IPhoneStyle on an iPhone 6 running iOS 9 results in your application being scaled incorrectly (it is about a quarter of the size it should be).
This bug is documented here:
https://forums.developer.apple.com/thread/13510
This can be fixed by turning off shrink-to-fit in the RenderHTML function of TTIWIPhoneStyle as shown:
parentform.ExtraHeader.Add('<meta name="viewport" content="width=device-width, shrink-to-fit=no, initial-scale=1.0, user-scalable=no, minimum-scale=1.0, maximum-scale=1.0" />');