Why on Iphone is the Instagram text box so tiny compared with Android?

The instagram text box on the Iphone is tiny - literally just 3 lines visable, it scrolls up and down but you really can't see much for editing. By contrast, on Android the text area become bigger as one types more - much better.

I've just visited the Apple Store and found that Instagram on the newest Iphones (15Plus, 15 ProMax etc) is just the same. As I use Instagram every day for my business, I keep using my Samsung while I'd really much rather be on the Iphone.

Can it be fixed?