The iPhone 6 vs 6 Plus is a perfect example of whatever you can do, I can do better. If the iPhone 6 has great battery life, the 6 Plus is still better. If iPhone 6 can take great photos, 6 Plus can take it better without jitter. If the screen on the 6 is beautiful, the 6 Plus is bigger, sharper, and just better to view content and faster texting like on SwiftKey since the keyboard can be taller and wider.
The only two advantages the smaller iPhone 6 has it is smaller for less hand gymnastics to reach certain parts with your one thumb, more pocketable, and lower price which is another reason it is still selling better. But the Note series have never really outsold the S series either although both are quite popular in Asia. I actually also believe the iPhone 6 design looks better for that specific size and more comfortable ergonomics. An IF those reasons is why people picked 6 over 6 Plus, more power to them. Nothing wrong with certain preferences to make a decision.
I really wish Apple can differentiate the two of them more similar to Samsung's S & Notes. Come up with different design schemes or exclusive features. The S5 is waterproof, the Note 4 isn't but has a more powerful SoC, S-Pen, and multi-task windows. Apple should use even more premium materials to the Plus series if people have to pay a premium to get the best version iPhone from a "technical standpoint." Give the 6s Plus line at least 3 GB RAM and bigger difference in battery life. Also improve the one-handed mode like what Asus and Huawei did. They can cram more things in a larger footprint.
But kudos to Apple. They finally listened to customers and entered the phablet market. The iPhone 6 Plus was a gamble. Apple could have just played it safe yet again but they wanted their marketshare to actually grow and they can NOT do that by taking smaller steps while watching their iPad sales declining. I know 6 Plus can easily turn off millions of buyers not used to that size and coming from a 3.5/4-inch iPhone and phablets isn't for everyone but Apple is now reaping the rewards in Asia where it could very much reach 50/50 between sales of big brother vs little brother by next year.