RichEdit: Textlimit is always 30000

I would like to display a larg text (more than 100kB) in a RichEditCtrl in a DialogBox.
As soon as I put this RichEditCtrl into a DialogBox the TextLimit of the Control is automatically set to 30000, even when I try to set the TextLimit with ::SendMessage(m_RichEditCtrl.m_hWnd, EM_EXLIMITTEXT, 0, 120000);
After this Line the GetLimitText() still returns 30000.

When I put the Control in a ChildWindow, the Textlimit is no question, everything works, but that is not exactly what I want.

Could anyone Help me?