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
>
> ___________________________________________________________________________________
> If your question is of interest to others as well, please add an entry to
> the Wiki!
>
> maillist : ntg-context@ntg.nl /
> http://www.ntg.nl/mailman/listinfo/ntg-context
> webpage  : http://www.pragma-ade.nl / http://tex.aanhet.net
> archive  : https://foundry.supelec.fr/projects/contextrev/
> wiki     : http://contextgarden.net
>
> ___________________________________________________________________________________
>