It is nice to compliment someone who has lost weight.
Jack runs into his old friend Jill who is looking very different from the last time the two saw each other, having lost a lot of weight.
Jack wants to comment on her new appearance.
Jack comments on how amazing Jill's new figure looks.
Jack tells Jill that she looked better before she lost weight.
Jill is visibly flattered by the comment and even goes as far to ask him out for drinks at a later time.
Jill is visibly offended, tells off Jack, and walks away.