Subscript scaling with Latin Modern Math

I'm using XeLateX, Unicode math, and the Latin Modern fonts (Latin Modern and Latin Modern Math). To me, it looks like subscripts in mathematical expressions are being properly (optically) scaled -- they are not merely small-size versions of regular characters, they are wider and have fatter strokes.

Here's some code

\documentclass{article} \usepackage{unicode-math} \setmainfont{Latin Modern Roman} \setmathfont{Latin Modern Math} \begin{document} Here is some math: $x_2=2$. \end{document} 

and the output

In traditional TeX, the subscript scaling would be done by using a completely different font, like cmmi7. But the Latin Modern Math font only comes in one design size (10 pt, I suppose). So how is the subscript scaling achieved?