I imagine in contact between Roman regulars and Barbarians it was the Romans plundering the dead barbarians. Just how major battles did the Barbarians win?, not that many. Also the fabricae would have been in well fortified cities, something barbarians would avoid for softer targets.
An interesting question is how well armored the troops of Germanic successor kingdoms were which presided over former Roman infrastructure.
It seems only the African Vandals who were a small class in a very wealthy region had metal armor for all warriors.
Ultimately a factor would be the availability of iron. This wiki article is interesting.
http://en.wikipedia.org/wiki/Roman_metallurgy
And 10 kg for ordinary mail, stronger double and triple mail could go over 20 kgs. Legionaries most likey wearing the heavy mail and Auxiliaries the light mail.
Another factor was cost, a good strong mail shirt may have been 12 solidi, a small fortune.
With at least 10% of the Empires resources going to the Roman military, about 8,000 tons of iron per annum would have available for the army which should have been enough to equip the entire army with mail.