Shrink window in GTK+ dynamically when content shrinks?

If I am not mistaken the automatic resizing of windows only only happens when elements are too large to be drawn. Additionally the set_default_size method only matters when first drawing the window and unless I am wrong is never used again. I would suggest using the resize method to set the window size. (link)

window.resize(box.width_request, box.height_request);

One thing you need to remember when using resize if you can't resize it smaller than the request_size if you run into that issue use the set_request_size method.