I use the Libertinus font family in my documents. Libertinus Serif contains superscripted and subscripted versions of Arabic numerals, whereas Libertinus Math don't.
In the math mode, for some reason those glyphs are used (leading to unpleasant kerning etc etc) with anything less than \normalsize, explicitly (that is, through \small, \scriptsize, \footnotesize, and \tiny) or implicitly (for example, through ^ and _). Wrapping the superscripted/subscripted numerals in \mathrm helps as a workaround. But surely there must be a better way?
\documentclass{article}
\usepackage{unicode-math}
\setmainfont{Libertinus Serif}
\setmathfont{Libertinus Math}
\begin{document}
$1234567890^{1234567890}$
$1234567890^\mathrm{1234567890}$
\end{document}


{\footnotesize $1234567890$}and{\footnotesize 1234567890}you can see the same effect. – egreg Jun 13 '16 at 15:03\normalsizeuses those glyphs... Let me rephrase the question to reflect this. – Taiki Jun 13 '16 at 15:07\smalldigits, which are larger than 10pt, still use the alternative glyphs. – Taiki Jun 13 '16 at 15:28\setmathfont[SizeFeatures={Size={-}}]{Libertinus Math}alone seems to work perfectly. – Taiki Jun 13 '16 at 16:12sstyfor everything: also letters are affected. – egreg Jun 13 '16 at 16:16sstyfor everything: https://cloud.githubusercontent.com/assets/15276544/16033841/4c2883ce-3206-11e6-9aa3-4e367394e662.png – Taiki Jun 14 '16 at 07:04