Should your font size change based on size and orientation of the device?

I am created a cross platform app and I was wondering if and when fonts should scale based on the size of the screen. Also, I was wondering if the scales of fonts should change based on the orientation of the device or on very large/very wide devices.

For example, I have one screen which has the text as the main subject of the screen. This font looks a bit odd on different devices when it doesn't scale.

On buttons, however, the font looks fine without scaling.

Also, there are certain titles that look fine with rotations and others that look atrocious.

Are there any guidelines about which, if any bits of text, should scale and based on what (ppi, size of phone)? I know the common web standard is rem, but that doesn't exist in mobile in certain cases (including mine).