I take it you're on windows? If so, then the best you can do is script your resizing and trigger it as the windows change focus, so that the parent window re-maximizes when it gains focus after the report window is closed.
The Windows OS version of FMP does not support multiple independent windows like the mac version does. Thus all "windows" are contained within the main application window, and subject to behavior as you noted.
You can use the OnWindowClose script trigger (file > file options > script triggers) to run the check to maximize the window.