I have noticed the same on Instagram, Strava, Garmin Connect, BOM Weather, Bank First, Shopping List, Coles Mobile Wallet, Rain Radar, which are all iPhone applications that just rescale (poorly) to iPad sizes.
I see 2 possible solutions.
1. IOS 12 should have a setting that allows it to change back to how these apps were sized in IOS 11.
2. The app developers should create iPad versions of their apps.
I can't believe that Instagram hasn't done this yet. Instagram is about presenting photos, personal art, it's supposed to be a visual based app, and yet they have only produced an iPhone version that won't scale. They have been polluting our instagram feeds with advertising for a while now, so they should have no excuse that they can't finance an iPad version of software.