The more you use an app, the more space the app takes up on your phone. So if you use Instagram more than other apps, it's likely it will take up more space on your phone than apps you don't use as often. However, your phone automatically resets the storage space an app uses every week, so you don't need to worry about storage for any app becoming gradually larger over time.
If you want to manually decrease the amount of space Instagram takes up, you can delete and reinstall the app (your photos and profile data will be saved).
On iOS, you can find out how much space Instagram takes up on your device by opening the Settings app and tapping General > Usage and finding Instagram in the Storage section.