Notice how Bucky is still pale and sickly looking from being experimented on. Now he's sitting *alone* with Steve, who saved him from all that. Peggy and the other girls pay him no attention anymore. Now he's just some ragtag soldier in the shadow of the great Captain America. But I don't think it was because Capt. was so great that he felt invisible. He felt different than before, but not because of anything that people could really see; just less confident, less *there*. But still smiling.