Equations surrounding font size

Hi,

I have a book with thousands of symbols and equations written in mathml.
I work on mac where calibre shows excellent using mathjax, with the symbols showing the same size with the surrounding text.

On windows calibre, readers complain that the equations show smaller than the surrounding text, really not so good...

My code everywhere is like that:

<p class="body">Equation is...
<span>
<math xmlns="http://www.w3.org/1998/Math/MathML" alttext="equation" xmlns:m="http://schemas.openxmlformats.org/officeDocument/2006/math"><mi>d</mi><mover accent="true"><mrow><mi>S</mi></mrow><mo>→</mo></mover></math>
</span>
</p>

Is there a way to adjust this calibre or code? Why is this difference between two platforms?

Thank you.

Elias