Instagram app narrow on iPad with iOS 12 update
The Instagram app still hasn't turned on iPad sizing, and in iOS 11 the Instagram app would use the aspect ratio of the iPhone 8 (whether actual size or ‘zoomed’) but with iOS 12 the app is now iPhone X aspect ratio, with black bars on the left and right.

It doesn't look like there is a way to return to the previous aspect ratio, similar to how one of the iOS upgrades changed the aspect ratio from iPhone 4s to iPhone 5 on iPad in the past.

As iOS 12 has only just been released, you can still restore back to iOS 11, but I don't think an aspect ratio change to a single app is worth staying on an old iOS forever for.