The latest version of GIMP, 2.8.6 ref-2, still shows the bug of the
"jpg-precalculation of file-size".
If you save an image as "jpg" a window pops up - and you are
asked what quality you want to have. In this window you can
check "precalculate jpg-file size". But this precalculation still
doesn't work. You always get a constant value "5.9 EB".
The precalculation-size is very helpful if you work on pictures
which had to be small. So if you send pics via e-mail or
you work on websites etc. you need the precalculation of jpg-
sizes in order to choose the appropriate compromise between
quality and file-size.
In my exploration in the forum "gimpforum.de" somebody
assumed that the portable version of GIMP was compiled with
the currend GIMP-version, but with an older version of the
GLIB. He said that most probably this older GLIB is the
reason for the wrong precalculation of the jpg-file-size.
So this problem should be solved by a new portable version
based on the current GIMP-installer version, which is here:
Could this be worth a try?