What makes the tendons large and feet thick?
Chris, I would be just as excited as you if western science and medicine were to elucidate the nature of the link between internal training and the way the connective tissue changes. But we don't yet have a mechanism, so there isn't that much point talking about it (I know there is still some point, but at any rate we won't get closer to mechanism by talking about it). The point is tendons are a critical type of connective tissue that may be specifically trained. Training results in changes such as thickening. The idea there is roughly that tissue that is trained to handle more load becomes thicker.
As to why it would show up in feet, I'm sure you've read things like Mike Sigman's blog and many people's posts here that talk about how the whole body is involved.
Anyway I think it is an interesting idea to think about the foot as a marker for development.