Hi, this is a great question. This is my best answer.
Joey: You look good.
Angela: That's because I'm wearing a dress that ACCENTS my boobs.
Joey: YOU DON'T SAY.
In this case, the word "ACCENT" = "to give emphasis or prominence to; accentuate".
Basically, Angela is saying that the dress in making her boobs more noticeable.
When Joey said, "YOU DON'T SAY." He was saying that he had already noticed, but didn't want to say anything.
Hope this helps you understand better.