There is probably a misconfiguration in resolution settings either at Unity or Itch.io.
I would first check the Unity build settings Build Settings -> Player Settings -> Player -> Resolution and Presentation. Then set the width and height resolution values to the amount of pixels you want to have.
Then check the same for Itch.io. In Embed option you should have Embed in page and Manually set size options selected. Then you can enter the same numbers (or slightly bigger numbers) for the width and height fields than what you have in Unity settings.
Let me know if this helps.