iPhone 5S vs 7 Battery.. what gives..

So I've decided to buy an iPhone 7, it's on iOS 11.3.1 (DFU Restored)

My iPhone 5S is on iOS 8.4

Battery Stats:

iPhone 7: 303 Cycles, 12% wear, apparently from Battery Info Beta

iPhone 5S: 277 Cycles

I charge them both up to 100%, below's the results.

iPhone 7 lasts 40 hours standby WITH low power mode off, Air Plane mode, etc..

iPhone 5S is 48 hours in, and 60% battery left. WIFI on, no settings changed or needed. Lasts over double the time.

So what's going on here..? Where's Apple's 10+ days Standby claim? Really tempted to resell because of this..

Usage times iPhone 5S wins by 15-20%.

iPhone 7 lasts 4 hours.

iPhone 5S lasts 5.

Why is iPhone 7 battery so bad?

Please forgive frustration.. but this is quite ridiculous to see an iPhone 7 claiming better battery efficiency, only to die twice as quick.

What can be done to fix it...