Font size is not currently supported by UI Toolkit.
See this thread for more information: https://forum.unity.com/threads/button-text-size-in-doesnt-work.1037551/
The feature is in our backlog, but no ETA at this time.
How to reproduce:
1. Open the attached "repro_project.zip" project
2. Open UI/CauldronUI/CauldronMainUI.uxml
3. Select .buttonSmall class and set it's Text > Size value to 100%
4. Select "Mix" button in the hierarchy and set it's size to 100% and observe
Expected result: Both text's inside the buttons should either fit the whole parent visual element or the borders of the button
Actual result: Small buttons text fits button borders, while Mix button text fits parent visual element