I personally don't believe that and think Mr Cook may have been using the opportunity to have a dig at Android by suggesting they are losing custom. I know upgrades in the UK were unprecedented this time around. I have heard very few people complaining about the size jump of the iPhone 6 apart from a few isolated threads on here. My wife briefly moaned about the increase in size but she adapted within days of using the device.
Out of interest how do they know how many people have moved over from Android? Do the carriers release information as to previous contract choices or is it a survey every iPhone owner is asked to complete when they purchase the device? I know I haven't been asked and if people switch carrier at upgrade time, I doubt information is passed on apart from a PAC code for taking a number with them. I think Tim has made an educated guess rather than relying on comprehensive data.