Why would your boyfriend ask what size panties you wear and then say nothing when you ask him why he wants to know?

Answer

Because he wants to buy you some lingerie as a gift, and wants to make sure he will get the correct size.

Maybe he tried yours on and they felt good and fit good but no size tag so he wants to know so he can buy the same size for himself

Different answer: I believe he wants to buy some as a gift, nothing is worse then buying someone you like underwear and finding out it is the wrong size.