Well i've not used the ios apps in a very long time, ever since i ditched the iPhone for Android, but on the Android version of the Kindle app, you can change the margin sizes and line spacing. I can't imagine that the iPhone/iPad apps are that dissimilar from the android app that they can't do this.
Yes you can change the margins and the line spacing in the iOS Kindle app. You can go from really stupid to quite stupid to stupid for the margins and the line spacing. Amazon leaves you with too much margin space and too large a line space. So what the user gets is a poor reading experience.