Major resolution discrepancy when comparing 2 Android phones. What could be causing this?

It depends on how your UI objects are set up. If they're all using fixed dimensions (like a button width being 50 pixels), then you can expect this to cause different results across different device resolutions.

You can specify a reference resolution to mitigate this on the Canvas Scaler component (which will be on your Canvas by default, but needs to be configured). In your case, you'd set the reference resolution to whatever your phone currently uses by selecting the Scale With Screen Size option:

To check how your scene will look on a known resolution (or aspect ratio) in-editor, you can define resolutions in the Game tab directly.