Is that because the frame is only as big as the button unless you manipulate it via something like -fill?
Yes. You can see that effect if you also set -highlightbackground and -highlightthickness: 
my $fA = $mw->Frame(
                -background => 'purple',
                -highlightbackground => 'maroon',
                -highlightthickness  => '1',
                    )->pack(
                        -side => 'bottom',
                        #-fill => 'x',
                            );

[download]

You will see a maroon border around the button - but that's the border of the frame. If you cycle the elements of that window with <Tab>, you will see that the black line around the button is 2 pixels thick - that is because the frame and the button are highlighted, visible if you add a -highlightcolor to your frame, of if you let it fill in x direction.

--shmem 

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}

In reply to Re^2: Get a Tk Button to align left in a frame by shmem
in thread Get a Tk Button to align left in a frame by ff

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