The wide margin is the default American size too and not a universal size..
It depends on your definition of "wide." The default left and right margins
are 1.25". The reasoning behind them is fairly simple: on 8.5" paper, those
margins give a 6" text line, which is 72 characters of 10-pt Courier New, a
common measure in typewriter terms. The margins are also reasonable for
(usually short) business letters, which were probably a major focus of early
versions of Word.
That said, anyone who is complaining about a "wide" right margin is not
likely to be referring to the default margin (regardless of how wide you may
consider it) because it is the same as the left margin. If someone is seeing
an *abnormally* wide right margin, the most likely cause is Track Changes
with balloons.
I would also take issue with your statement that U.S. letter is "much
shorter than the universal A4 size." Letter is 11" long, A4 11.69"--or
metrically, 27.94 cm for Letter vs. 29.7 cm for A4.

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Suzanne S. Barnhill
Microsoft MVP (Word)
Words into Type
Fairhope, Alabama USA
Word MVP FAQ site: http://word.mvps.org
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> The wide margin is the default American size too and not a universal size..
>
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> >> > Your
> >> > help is appreciated.