Thanks, IMO that's a reasonable request. It's here now:
https://bugs.launchpad.net/python-mode/+bug/1266169
After further reflection, the following result: Shrinking the output-buffer only has effect for a very limited number of events. Afterward the buffer is filled and window-size is half-way like the current default.
From this don't see it's justified to hard-code it.
Also there is a solution, which seems to come nearby the desired behavior:
M-x python RET
switch to py-shell resp. output-buffer and
M-x shrink-window-if-larger-than-buffer RET, it's on C-x -
Than M-x customize-variable RET py-keep-windows-configuration RET
to `t' or 'force
The following run should keep the window small, resp. enlarged.
To make customization easier, extended the menu: Python/Customize/Switches/Display/py-keep-windows-configuration