I can think of a couple ways that the characters' personalities are reflected in their physical characteristics.
Julia, first of all, is a very fit, healthy-looking, and youthful woman, which is probably a strong reflection of her vital personality and great inner strength. On the other hand, her appearance of strength can be misleading. In fact, Winston at first believes that Julia is perfectly happy with her life in a society controlled by the Party and Big Brother.
Winston, on the other hand, is much less physically fit than Julia. In fact, he is plagued by various health concerns, and is of relatively small stature. This probably reflects his helplessness against the Party, and contrasts his emotional and mental weakness against Julia's strength, which allows her to set up a very convincing facade of contentment while at the same time she manages to enjoy the pleasures forbidden by the Party.
O' Brien's strong, solid appearance also probably reflects his authoritarian persona. His physical appearance also seems to have belied his true personality, however, because Winston at first believes that O'Brien is a man who can be trusted and looks to him as a leader. However, as it turns out, O'Brien was not to be trusted, and in fact was an important figure in the Party.