I've got a plain old IPhone 4, so the size and weight aren't issues. However, I prefer to keep my phone in my jersey so I can reach it to take photos, answer calls if expecting something important, view maps if needed, etc. I also commute year-round, and stashing the phone in my pocket keeps it warmer than it would be in a seat bag. That said, there are a lot of nice seat bags large enough to hold an IPhone 6+. I have no qualms using a larger seat bag since I bike commute anyway. Topeak and Jandd make some nice larger bags that will fit under a saddle. A Carradice Barley or their zip roll bag would easily hold your phone, and they have a nice classic appearance. Acorn bags are the nicest but good luck trying to buy one.