I think there are multiple things going on and people need to be more precise in how they're using their camera.
For example, to respond to P_D_R's post (right before mine), I don't use iCloud Photo Library, so the "Optimize iPhone Storage" isn't even available for me. If I turn iCloud Photo Library on, "Download and Keep originals" is the only option checked. I instantly turned off iCloud Photo Library when I first got the phone, and I've had terrible pictures the whole time. My kids faces look like make-up barbie doll artificial faces.
There are other factors as well. I have HDR turned off all the time as well. I have flash turned off all the time.
My theory is this: Every time Apple bumps up the megapixels on their cameras, they increase the JPG compression/noise reduction in order to keep the image size down. I have noticed no .jpg file size increase in the photos taken on my 6 plus vs. my 7 plus...and there SHOULD be! I've gone from an 8mp camera to a 12mp camera...the file sizes should be bigger. But I think Apple doesn't want people complaining that pictures are eating up all the storage on their phone.
I'm keeping my 6 plus on iOS 9 (I read elsewhere that someone is experiencing this with any Apple device which has a 12mp camera and iOS 10), but I really really hope Apple addresses this.