org-src-window-setup

org-src-window-setup is a variable defined in `org-src.el'.
Its value is

reorganize-frame

Documentation:
How the source code edit buffer should be displayed.
Possible values for this option are:

current-window Show edit buffer in the current window, keeping all other
windows.
other-window Use `switch-to-buffer-other-window' to display edit buffer.
reorganize-frame Show only two windows on the current frame, the current
window and the edit buffer. When exiting the edit buffer,
return to one window.
other-frame Use `switch-to-buffer-other-frame' to display edit buffer.
Also, when exiting the edit buffer, kill that frame.

You can customize this variable.