Thanks Taco, this workaround works for me:

\Umathfractiondelsize\textstyle = 12.12pt
\Umathfractiondelsize\displaystyle = 28.68pt

Lutz

2009/7/3 Taco Hoekwater <taco@elvenkind.com>
Lutz Haseloff wrote:
Hi Hans, Hi Taco, Hi all,

a couple of days ago I complained about not adapted delimiters around
constructions with \atopwithdelims.
With LuaTeX, Version beta-0.41.0-2009070222 (Web2C 2009), this
problem persists. After reading the last luatexref-t.pdf I tried following:

\starttext
\Ustartmath\Ustack {a \atopwithdelims() b}\Ustopmath
\startformula
\Ustack {a \overwithdelims[] b}
\stopformula
\stoptext

This is a known problem. Sorry I missed your message earlier. The
details of this problem are here:

 http://tracker.luatex.org/view.php?id=206

Best wishes,
Taco
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