Aren’t open access publishers niche publishers that don’t publish much?

No. As of 2016, Public Library of Science had published over 200,000 articles. BioMed Central – nowadays part of Springer Nature Publishing Group – publishes more than 290 renowned open access journals.

In addition to the well-known open access publishers, large international publishers and specialist associations are establishing their own open access journals. These include the following publishers: Elsevier, Wiley-Blackwell, Sage, Royal Society Publishing, Taylor & Francis and American Physical Society.