I don't have Tk on this machine to test this with, but you may want to try creating your widgets with -expand => 1 to expand the allocation rectangle to fill all the unfilled space, and -fill => 'both' to make the widget take up the full space of the allocation rectangle.

That's only for the pack manager, not for the grid manager. For the latter, you need to use the -sticky option, and set it to the value nesw.

Abigail