An article in business insider speaks of a rumor of two large sized iPhone offerings. It also mentions a gigantic iPhone. A brief look into the reason why a gigantic phablet might hurt for Apple.
More bigger the iPhone, bigger the cut in the apple's apple logo
The cut in the logo might literally turn out to be a cut in sales of iPad, if Apple ever turns out to make a phablet similar to note series. The desire for the iPad tablet drastically reduces if there are phablets with iOS ecosystem kissing the 6 inches screen with retinal display. Apple could have had a forethought of this when they persisted with a modest increase in screen size for iPhone 5 and subsequent 5S and 5C.
Apple : A product centered company
Apple desires to maintain difference between its devices. If you want to a phone with smartness, go for iPhone. Want a consumption device, go for a iPad. If you want productive ones, go for MacBook or iMacs. Apple has to maintain these boundaries if it has to reap in more profits. The product centred culture profits from such boundaries and as far I feel they will continue to demarcate this line rather than blurring the lines as the Surface is trying to do with its Pro offerings.
Link to article in Business Insider