By default your browser is supposed to chose the optical size value of a variable font automatically based on the actual font size.
Currently every major browser engine is doing it wrongly. To
illustrate this, below are two specimen of a waterfall view. As a proof, the red
overlay is setting opsz correctly (in pt) and explicitly using the CSS
font-variation-settings property. The black text is the actual test.
If the red and black are different, you browser is still broken in this regard. Good luck!