http://www.sealiesoftware.com/blog/archive/2013/09/24/objc_e...
and here:
https://www.mikeash.com/pyblog/friday-qa-2013-09-27-arm64-an...
Basically, as I understand it, 64-bit pointers on iOS can actually act to decrease memory pressure because the extra bits can be used to store object class information that would otherwise have to be stored separately.