Symbolically you can say that Obi-Wan became more set in his ways as he transitioned from TPM to ROTS, that he became more constricted by his beliefs and less able to stray from the Jedi Code. His costume develops more layers and thicker fabric and it’s bound very tightly around his throat. It looks rather heavy and stifling and suffocating.
However, if you take CW Kenobi into account, it kind of makes that transition from AOTC to ROTS a little harder to understand. He’s less fanatical in The Clone Wars; you can see little moments of Obi-Wan’s conflict – there’s a lot of “do as I say, not as I do” in CW Kenobi.
As a writer I guess I see his costume in 2 different ways. In ROTS Obi-Wan is a Jedi Master in his prime, wearing all the symbolic pieces of a powerful uniform. It’s bigger and more imposing because he is bigger and more powerful than ever. However, he has been through incredibly painful experiences. And though he has remained true to the Jedi Code, the harder he clings to his ideals, the more suffocating and toxic they become. The very fact that his tunic creeps higher and higher up his neck makes me feel almost as if it were going to choke him.