Lost among the ribbon cuttings and public announcements heralding Amazon.com Inc.’s fast-growing portfolio of sorting and distribution centers is the company’s quiet expansion in some of the country’s strongest technology hubs.

Amazon’s low profile in those cities is not an accident.

About 25 percent of Amazon’s corporate workforce of some 382,000 employees is spread throughout a dozen or so research and development locations outside of its home in Seattle, according to sources familiar…