I’ve been thinking about this since I posted it, and I came to similar conclusions. There are a cluster of behaviors that seem to mean discomfort and therefore low status: fidgeting, jumpiness, talking too fast, certain eye contact patterns (staring at the person and then looking away fast when they turn to look at you), ums and ers.
Some of them feel hard to disentangle. Whether you hold your head high seems mostly about status, but also a little about size. This seems like it might be inherent in the territory: There’s a fine line between credibly signaling that you’re powerful and implicitly threatening to use that power. (Schelling’s The Strategy of Conflict comes to mind here.)