jargon.artrocity.com
New Hacker's Dictionary the Jargon File v 4.4.6
[uncommon; prob. from the nuclear military] An unexpectedly large quantity of something: a unit of measurement. “Why is /var/adm taking up so much space?” “The logs have grown to several barns.” The source of this is clear: when physicists were first studying nuclear interactions, the probability was thought to be proportional to the cross-sectional area of the nucleus (this probability is still called the cross-section). Upon experimenting, they discovered the interactions were far more probable than expected; the nuclei were “as big as a barn”. The units for cross-sections were christened Barns, (10-24 cm2) and the book containing cross-sections has a picture of a barn on the cover.