http://www.sciencedaily.com/releases/2006/12/061222092912.htm 
[...]There are several potential explanations for these findings, say the authors. For example, surgeons spend a lot of time in operating rooms, which are cleaner, cooler, and have a higher oxygen content than the average medical ward, where physicians spend most of their time. They also often wear clog-type shoes that adds 2-3 cm to their perceived height.
In contrast, physicians have a tendency to hang heavy stethoscopes around their necks, which bows their heads forward and reduces their perceived height.
[...]