With the new Edge plans and payment plans for phones, it's making the larger capacity phones more affordable for just a few bucks more a month.
I always buy the entry level, minimal storage iPhone. However, I'm noticing with my 6 Plus it's no longer cutting it. I'm playing more games due to the larger screen, taking more pictures, taking more videos. Granted, iCloud does help with photos/videos but many games are becoming 1GB+ (Mortal Kombat) and many apps are storage-hungry (Numbers/GarageBand/Etc).
Might be the first iteration of the iPhone I opt for a larger size