I’m using an S7 and have been using trading 212 for many years. My total size is under 200mb. It’s probably just a problem with your cache. Clear the cache, and all may be resolved. If that fails then uninstall the app and reinstall it. Clearing the cache will not delete your data.
To clear the cache, go to the phone’s settings, select apps, trading212, storage, clear cache. Easy as that.
You can do that with pretty much all the apps on the phone, and may be able to save significantly more storage space. And as I say if it doesn’t work then reinstall the app. If that doesn’t work I haven’t got a clue. All the best.