Reproduction steps:
1. Create a new project
2. In the Project window, create a new UXML document by pressing the right-click > Create > UI Toolkit > UI Document
3. Open the newly created UXML document with the UI Builder
4. Select it in the Hierarchy of the UI Builder window
5. In the Inspector of the UI Builder window, check the “Match Game View” property under the “Canvas Size” category
6. Dock the UI Builder window anywhere in the Unity Editor
7. Observe the “Size” property above the “Match Game View” property (the Game view resolution and the “Size” of the UXML document should be matching)
8. At the top-right of the Editor section where the UI Builder window is docked, press the More Items (⋮) menu > Maximize
9. Observe the “Size” property again

Expected result: The “Size” property of the UXML document has not changed
Actual result: The “Size” property of the UXML document has changed to 640 x 480

Reproducible with: 2021.3.34f1, 2022.3.17f1, 2023.2.6f1, 2023.3.0b3

Reproduced on: Windows 11 Pro (23H2)
Not reproduced on: No other environment tested

Note: Unmaximizing the UI Builder window returns the “Size” property of the UXML document back to matching the Game view