Thanks to your gentle prompting, I believe I've found the answer, from man Tk::Wm:

GEOMETRY MANAGEMENT By default a top-level window appears on the screen in its natural size, which is the one determined internally by its widgets and geometry managers. If the natural size of a top-level window changes, then the window's size changes to match. A top-level window can be given a size other than its natural size in two ways. First, the user can resize the window manually using the facilities of the window manager, such as resize handles. Second, the application can request a particular size for a top-level window using the geometry method. These two cases are handled identically by Tk; in either case, the requested size overrides the natural size. You can return the window to its natural by invoking geometry with an empty geometry string.

We'll see... :D