Wiat until she starts complaining about clothes. Since men's clothing is based on measurements, and women's clothing is based on nothing, this is a fair complaint.
Say things like "Yes, but buying men's underwear is like buying a bra based on how tall you are" or "Yes, but men's underwear is based on your waist size, which is totally ridiculuous. If I want something around my waist, I'll buy a belt"
You'll then likely have a joke together about how men's underwear would have to be sold as large, larger, and largest, because most guys would rather do without than buy a small.
It won't take her too long to realize that a) some men don't complain about this and b) many men don't have this problem. She'll put 2 + 2 together, trust me. Almost every women I've ever met appriciates subtle.
And yes, those jokes have worked.