`\mathlarger` is not currently support by jgm/texmath, the library we
use for tex -> docx math conversion. Nor is there a good prospect of
supporting it soon, since the underlying equation model doesn't include
sizes. I'd recommend adding a latex macro redefinition for this to
your document
\renewcommand{\mathlarger}[1]{#1}
This will allow pandoc to compile it, though the size won't be larger.