When I switched from the iPhone 6 to the 6 Plus, I made a point about the battery:
I feel like I'm doing something wrong. I am getting marginally better battery life since switching to the 6 plus — not dramatically better as is the norm. I even did a fresh install this time.
What I hadn't yet realized was the reason. It's the screen, stupid. The fluorescent lights at work make my phone's screen hard to read. My brightness is set between 75% and 100% while I'm at work so I can see. Given the fact that the 6 Plus has significantly more pixels to power, it makes sense that a high brightness level would cancel out any battery life gains from the larger battery.
On Sundays, the battery life is superb - much better than my 6 was. I also keep the brightness at a reasonable level.