MathJax doesn't any larger sized arc's in its fonts, so the \frown character is not stretchable past its normal size. But you can use an overparen, which is stretchy. I already gave you a link to the discussion that shows how to do that. Here is another way:
\overset{\mmlToken{mo}{⏜}}{XYZ}
The STIX fonts include several sizes of the overparen (though the bounding box data in the font isn't correct, so one size is placed a bit too low), but with the MathJax web fonts, they will be flat in the center most of the time. Not exactly what you want, but the best that MathJax can offer at the moment.
Davide