I am reopening this problem that has been posted for a long time now. That post is titled "S8 Full Screen Instagram"
It has been marked as solved by one of your moderators and it clearly hasn't been fixed because people are still posting on that thread up to now (it's been 2 or more years I believe).
The problem is that when we view Instagram stories the sides are cropped off by the phone and I think it has to do with the screen ratio. We cannot fix this ourselves however because Instagram is marked as optimized for fullscreen. It clearly is not and It's annoying to have a thousand dollar phone not optimized for these kinds of things. (I've also noticed this on Facebook, havent noticed it on Snapchat)
Please Samsung stop ignoring this problem and try to work with those app creators to fix this. I've tried to make a report to Instagram but they ignore me too.