Not sure if this is the issue but I had this problem but if you go to each of your canvas's and go to canvas scaler change the settings to mine if your base resoultion is 1920 x 1080,then set UI Scale mode to Scale with screen size your resoultion you building your game in and then Scren match mode set to match width or height and set match to 0.5 you can try that and then export and upload to a test project on itch and check if correct
Also for webgl I go to project settings in player and then resolution and presentation set default Canvas Width to 960 and default canvas height to 540.
You can try this hopefully solves the issue
