This is a short post with some info I searched for when considering a Fenix 6S Pro during Black Friday.
I used a Forerunner 735XT for a long time, after a 645 (which was practically worthless), and a 230 and a 235, which I loved. The only thing I couldn't really convince myself of was the weight of my Forerunners, which are very light indeed, and the added weight of a Fenix. I opted for the S version, as I have small wrists and it is the lightest version.
First, the weight of the 735XT is 40 grams.
Second, the the weight of the Fenix 6S Pro is 60 grams.
I have weighted both watches multiple times, with the same results. Both watches have the default Garmin straps on them.
Conclusion: the Forerunner 735XT weighs 2/3 or 66% of the Fenix 6S Pro. However, I don't really feel it while running or wearing it during the day. The only time it is noticeable for me is while in bed, but it doesn't really bother me.