I guess the larger display drains the bigger battery on the 7+ More, which is why the battery life is only a little better than the standard 7.
Or perhaps that's more of an average (or even slightly below average) battery life of a 7+ compared to above average battery life of 7.
I think what apps you use and the brightness make a big difference. I've seen higher battery life but when I'm on high brightness, using Tapatalk, Facebook, Safari, I'll notice quicker battery drain.