Nested DataGrid Problem in Silverlight

Question

I have a data grid, in each RowDetailsTemplate I have one Expander with Content of expander is another datagrid, when the user wants to see the dataGrid List, he clicks on the expander to open and doing some functionalities.

The row height automatically grows when the expander is expanded but when the expander is collapsed back, the row height doesnt shirk. Can anyone pls tell me how to set the row height back after the expander is collapsed?
I have spent much more times to solve this problem. But still I did'nt get any solution...
Kindly any one help me...
Thanks in Advance.