iPhone 5S vs 7 in battery performance. 7 sucks, why?

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.

iPhone 5S is 80+ hours standby, I had to use it, though had 24% left.

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, or make it more efficient?

I've tried turning off background app refresh, email push, bluetooth, etc.. soon to get a battery replacement, though if standby times are this crap.. what's the point? Could a battery replacement truly help this issue? (It is after-all 2 years old)

Looking for a scientific answer here somewhat..

Thinking I should resell and get a 7+ instead.