How to reproduce:
1. Open the attached project
2. Open the Sample Scene
3. Go to the Hierarchy window-> Select Canvas
4. Go to the Inspector window-> Canvas Scaler (Script)
5. Set UI Scale Mode to the Constant Physical Size
6. Inspect the Game view when setting different UI Scale Modes

Expected: UI is the same as it appears Windows with Constant Physical Size
Actual: UI is three times bigger with Constant Physical Size on macOS, in the Editor and Player

Reproducible with: 2018.3.14f1, 2018.4.0f1, 2019.1.2f1, 2019.2.0b1, 2019.3.0a2

Note: Could not test on earlier versions because of the thrown errors