Yesterday was new bike day 
Picked up a SuperSix Evo that came with a 160mm rotor up front and a 140mm out back. I’m a big guy at 6’5" 230ish lbs (195 cm/104 kg) and have always had 160mm rotors front and back. I actually have a 180mm up front on my Cutthroat because why not 
Do you think I should swap out the rear to a 160mm? Most of the braking power should come from the front so maybe I can get away with the 140mm?
Thanks!!
I’m a similar weight right now and have 160/140 rotors, and don’t feel like I am lacking in braking. I mostly ride road and “champagne” gravel.
I think manufacturers are prioritizing front brake performance rather than the rear. It seems contrary to me too, but looking at motorcycles, they often have 2 discs up front for ‘added stopping power’, so 
The majority of your braking force comes from the front, so I wouldn’t rush out to make the change. Ride it a bit and see how it feels. You’ll know pretty quickly if you want a 160 in the back.
I’ve done it with all our bikes, but that’s mainly because we can then swap wheels with no compatibility issues, and don’t need to keep 2 sizes of rotor handy to replace them. I’m sure the braking would be fine on 140.
The downside to a bigger rear rotor is makes it easier to lock up, especially when braking forcefully in the front, which transfer weight off the rear wheel. It also makes it harder to modulate. Do you fine the 140 lacking in some way that a larger rotor might improve?
I’ve always been told bigger rotors mean better stopping power, and being a bigger rider, I never used a 140mm before to be honest. I won’t get to take the big for a proper ride until tomorrow.
I never had a problem stopping with 160mm rotors and Ultegra brakes on my last bike but I didn’t know how much the rotor size would matter.
“Stopping power” can mean different things, but usually means a brake with more stopping power will take less lever effort to achieve a given amount of braking. In the rear, how much braking can be achieved is limited by rear tire traction. Assuming you can already lock up the rear, a larger rotor will make it easier to lock up. There is another factor. On long steep downhills, dragging the rear brake can cause it to heat up. A larger brake will heat up somewhat less. Best, of course, is not to drag brakes and to use both the front and rear brakes.
Might as well go all the way up to 622s! 
As others said, fronts do most of the work, feel it out and see if you need to upgrade.
With the same pad, I don’t think actual stopping power would be any different.
As a thought experiment, the little 1 inch square rectangle pad is gripping the rotor and creating friction to slow you down. Whether it grips a 140 or 160, the friction is going to be the same.
What’s different is that 160mm spreads the heat dissipation over more surface area. With that you can brake longer before you experience fade. My guess is that you’ll never brake hard enough or long enough with a rear brake to experience that fade.*
I’d just ride it. You can always upgrade the next time you swap pads.
*Many years ago this was explained to me in car terms. People pay big bucks for some Brembo brake kit for their car. Assuming the same pad, these fancy brakes don’t stop the car any better than stock rotors. What they do is dissipate heat better allowing repeated stops without fade like on a race track. On a street car, Brembo brakes may look cool but they stop the same and are massive overkill.
If you’re only doing it for the stopping power then no, I wouldn’t. You can lock up a rear wheel just fine with the 140
I’m same size as OP. Never had issue with 160/140.
I think we need to see pics of the bike before commenting 
You’re not wrong Doing some tweaks today and hope to get my first ride in tomorrow. There will be pics!
It is never a bad thing to overspec brakes. I run 160 mm front and rear on my road bike and 180 mm front and rear on my XC hardtail (although I do regularly pull a trailer with it).
Given your weight, I’d definitely upgrade your rear brakes. The main two advantages are
My bike came with 140mm and when braking downhill it would smell like something was burning . I changed both to 160mm and no issues. I,m 181cm and 90kg
~16% more braking torque with 160 vs. 140 and rear break is ~20% braking power approx
So the change will increase your braking power by 0.16*0.2 = 4%
I went for 160/160 when I started riding in Spain more, due to the long descents.
In the flat UK, 160/140 would be more than adequate.
What tyres are you running on that - my suspicion is the tyres will lock up before you get to the point of noticing a difference between a rear 140 and 160 on sub 35mm tyres.
100% go 160. I love 140mm rotors, but burn them black far too much on big hills. I weigh 220 at 6’4", and with steep sustained 1000+ foot gravel descents, I could feel the hub shell getting so hot, that I was pretty sure it caused my bearings to fit too loosely in the shell. That hub is now filled with Loctite to get the bearings not too wobble. (there could have been other factors here, so could be wrong on this, but the bearings used to fit tight, and now they dont). For what its worth, power was always fine with 140, but heat was not.
Also, I understand the most the power should come from the front theory. But I have switched to bigger rotors on the rear as of late (enduro and XC bike), and its been a lot better. I wouldn’t do this on a gravel bike, as the braking is more similar front to rear IMO, I used to burn up 140mm rotors equally front and rear, but I’m just saying maybe don’t jump to that cliché. Maybe this is a personal thing, but I think it comes down to the type of braking that occurs on MTB’s, and your body position. Folks are going to jump to “your doing it wrong” here, but I feel pretty confident in this.