mh-e message composition behavior -- why split the screen?

[J G Miller]

When I wish to compose a message in emacs with mh-e, 99.9% of the time,
I am not viewing the MH-folders list but have a single pane displaying
the current buffer, either scratch or the file I am editing.

I then enter <CTL>x<CTL>m and enter the to address and then the frame
splits into two for no good reason rather than displaying the message
composition buffer in the whole of the frame.

How do I get the message composition mode to stop splitting the frame
and display the message composition in the whole frame without having
to enter <CTL>x1 for every message I wish to compose?