Dear Otared,
I think there is an issue with the size of math fonts in \frac when used in math alignments, as it is is shown in the following example.
I confirm what you said. However, if I use \dm{} instead of \displaystyle as following, then the output is correct. \placeformula \startformula \startalignedcases \NC - \Delta u \NC = 0 \NC \qquad\text{in }\, \Omega \NR \NC \dm{\frac{\partial u}{\partial {\bi n}}} \NC = V u + \phi \NC \qquad\text{on }\, \partial\Omega. \NR \stopalignedcases \stopformula or this (with\type{\displaystyle} added) \placeformula \startformula \startalignedcases \NC - \Delta u \NC = 0 \NC \qquad\text{in }\, \Omega \NR \NC \dm{\frac{\partial u}{\partial {\bi n}}} \NC = V u + \phi \NC \qquad\text{on }\, \partial\Omega. \NR \stopalignedcases \stopformula In this example, it is OK to use \dm inside of \startformula … \stopformula. I am not sure that it is what Hans and Mikael want. Thanks, Best regards, Dalyoung