A followup to my problem
Having read the Wikipedia article which describes the GIF file format, I
can see how an Image Descriptor could be used to reduce the size of a
GIF file by eliminating all but that portion of a frame that changes.
This allows the Logical Screen Descriptor to define a larger canvas. So,
without reading an entire GIF file, one cannot know the true extent of
an image since each Image Descriptor could provide a different portion
of the whole.
Since Tcl probably does not do this, it must return an image with the
size of the Logical Screen Descriptor -- in my case 114x114 (although
where ImageMagick came up with 114 is a mystery).
I have found another ImageMagick option that fixes the issue for me:
magick convert -dispose previous -delay 10 -resize x100 \
+repage F*.png -loop 0 T.gif
The "+repage" eliminated the unused portion of the Logical Screen. The
output file T.gif is now 87x100 for the Logical Screen and all Image
Descriptors. Tcl now returns 87x100 images for the image frames.