Enable "MathML (experimental; no images)" in Special:Preferences → Apperance → Math. Then preview a page with the contents <math>{n \choose k}</math>. The parentheses of the binomial coefficient are too small (see image below). If you omit the braces (i.e. <math>n \choose k</math>) then "k" is too small.

(screenshot from bottom of https://www.mediawiki.org/wiki/Extension:Math/Native_MathML/Reported_Cases)

Contrast this with <math>\binom{n}{k}</math>, which always looks good. I would expect that all three ({n \choose k}, n \choose k, and \binom{n}{k}) generate the same MathML code.