3500 mah battery for a 6.2 quad HD screen is just not enough. IMHO, it's not even close to being enough on a Samsung flagship. I'm sure we will hear how power efficient the new chips/processor are, but that won't do much to give us users a consistent 6+ hours of SOT over a an entire day. I have the s7 edge from April 2016 - 5.5 quad hd with a 3600 mah battery. I have turned a ton of things off and can just barely hit 5 hours sot with about 8 or 9 hours of total run time before it needs to be charged. Also, battery life has gotten progressively worse over time.
As far as the 7 plus is concerned, yes, it does have a 2900 mah battery, and yes the screen is also 5.5 inches like the s7 edge, but it's full hd, not quad hd. More importantly, Apple optimized the hell out of its software and hardware, unlike Samsung/Google etc. I am by no means an Apple fanboy, and while I did have the 7 plus for a few months and can attest to it having the absolute best battery life out of any phone (Huawei Mate 9 included), it's a boring phone otherwise.