The size-adjust descriptor in @font-face allows scaling of glyph sizes for a particular font face without affecting the CSS font-size and derived metrics such as em. CSS font-size can be seen as a scale factor for a box that the font draws in. Glyph sizes within that box vary between fonts, and size-adjust enables harmonising them across different fonts. That's why it can also help with reducing cumulative layout shift by matching up the fallback font and primary web font using this descriptor.
Harmonize perceived size across fonts that take up a different size within their em box. Reduce CLS by matching size and average width of laid out text between primary web font and system fallback font.