According to your description, the man said, "You are so thin". It should be regarded as a compliment, because praising a girl's thinness is usually an affirmation of her figure, but the specific meaning needs specific analysis.

If a man says "You are so thin" with a serious and sincere attitude, it is a compliment. If he said this in a joking tone or with irony, it may not be. In fact, whether you are thin or not, you should know that the way and tone of the other person's speech can actually explain many things.

Good luck, please accept it if you are satisfied.