When you say 'heavily armored' what you're talking about is generally
a vertical panel of steel plate about 3/8 inch thick behind the
pilot's seat. weighing perhaps 135 pounds (if 2' by 4') The rest of
the airframe weight was heavier construction to meet carrier operating
requirements. I have seen a Zero replica under construction and that
airframe was deliberately made as light as possible. Note that the
wing fold consisted of about two feet at each wingtip - designed that
way to save weight. The end result is the many cases of Zeros
disintegrating under 50 cal fire. When have you ever read of a Buffalo
or a Wildcat coming apart like that? Note that armor was added to
later model Zeros. Also note that about 80% of all fighters shot down
were shot down by people sneaking up behind them.
Walt BJ