My girlfriend bras are too big. Help

My girlfriend is 23 years old. She is a very attractive slim blonde. when the weather is nice outside she always wearing low-cut Shirts or low-cut crop tops. She looks wonderful in them, but there is one problem. All her bras are too big for her small (but very nice for me) breasts. The gap between a bra and breasts is so big that you can see her nipple. Many times I saw guys looking at her breasts. It pissed me that her nipples were visible to them. She has big puffy nipples and in a good light they are very obvious. It seems to me that she does not realize it.
It would be hard for me to count how many guys saw her nipples. Should I talk to her about it?