FALUN, Sweden—Sverker Johansson could be the most prolific author you've never heard of.

Volunteering his time over the past seven years publishing to Wikipedia, the 53-year-old Swede can take credit for 2.7 million articles, or 8.5% of the entire collection, according to Wikimedia analytics, which measures the site's traffic. His stats far outpace any other user, the group says.

He...