Steps to reproduce:
1. Open "BugDemo" project
2. Open "BugDemo" scene
3. Select Hierarchy>Canvas>Text GameObject
-- Notice that there is a huge gap between "Size tags makes new lines" and "behave really strange" text lines. Also
4.Change Text component's font size to 150
-- Now the gap is smaller
5. Set <Size=*> to Text component's font size or smaller
-- Now gap doesn't change

Results: <Size> tags in a UI.Text element creates gap between lines if the <Size> is less than the component's Font size

Reproduced with: 5.6.0a3, 5.5.0f1, 5.4.3p1, 5.3.6p7