Quoth the Geekend:
"Not since the maiden flight of Challenger in 1983 has a space shuttle operated with just a four-man complement. This reduced crew size can accommodate the extra payload on STS-135, but the payload isn’t the reason Atlantis is flying with the smallest crew in 28 years. That’s just a bonus."I divulge NASA's secret in efficient text form via the Geekend, but go into more auditory detail on TechTalk. The latter podcast also includes an unrelated interview with the authors of The Techno-Human Condition (affiliate link), which is a fine primer on transhumanism. It's worth a listen above and beyond my trivial contributions.