Using delimiter scalers (\big, \Big, etc.) within superscript-like environments

I have the following code:

$q \xrightarrow{\Brack{(i,a)}} q'$

where \Brack is defined using mathtools as

\DeclarePairedDelimiter\Brack{\lBrack}{\rBrack}

which yields:

However, I would like to scale the outer delimiters to be larger than the parentheses, using the usual \Brack[\big]. This however leads to them being as large as they would be if \big was used in a normal-sized text, making them far too big for the superscript environment:

How can I scale the delimiters to still be proportional to their superscript size and not to their normal text size? I am using unicode-math with lualatex.

Here is a complete MWE:

\documentclass{article}\usepackage{amsmath}\usepackage{mathtools}\usepackage{unicode-math}\DeclarePairedDelimiter\Brack{\lBrack}{\rBrack}\setmainfont{Tex Gyre Pagella}\setmathfont{Tex Gyre Pagella Math}\begin{document}\noindentWithout scaler: $q \xrightarrow{\Brack{(i,a)}} q'$ \\With scaler: (too large)$q \xrightarrow{\Brack[\big]{(i,a)}} q'$\noindentCompare:$\Brack{(i,a)}$ and $\Brack[\big]{(i,a)}$\end{document}