Not sure why the Tab Bar just keeps getting smaller and smaller. Surely it should be roughly the same size on iPhone 4S as it is on iPad? But the font is so small it can't be seen and the icons despite having @2x and @3x at the recommended 75x75 sizes they are also way too small on iPad. Not to mention that whole bar itself is much, much thinner on iPad. Its also too small even on iPhone 6 plus never mind iPads.
Edit: For some reason this post has been edited to show 'Tab Bar' rather than TabBarIOS but it is not the generic tab bar in React Native as that is different and behaves differently. Its very specifically the TabBarIOS module.