If you were an adult when the original Star Wars trilogy was new, you understood intuitively why Leia was in a bikini at the beginning of Return of the Jedi. Jabba the Hutt commanded it to humiliate and dominate her- to drive home the fact that she was his slave. That costume made it all the more satisfying when she killed him (at least for the adult audience). Carrie Fisher was all for it, after wearing a baggy gown in the first movie and pants in the second film.
Sculptor Richard Miller tells us the bikini was a way of showing how Princess Leia "grew up" over the course of the three movies. Yeah, that's BS; she was an adult in the first movie. It had more to do with how the little boys who loved Star Wars in 1977 grew up by the time Return of the Jedi came out in 1983. In that, the costume succeeded beyond their wildest dreams, as we are still talking about it 40 years later.