I found an answer to my own question. For anyone else that gets confused:
tabSize refers to the number of columns a tab takes up. It does not effect the amount of indentation.
This differs from other editors you may be used to, the “Tab Size” setting in VS Code for example.
If you are not using tabs at all you want to change the number of spaces used to indent, you want the indentUnit facet. More detail here: CodeMirror 6 set indentation unit