Tell her that it makes you happy. She wants you to be happy, right? If that doesn't work, buy some cheaters that make them look a size or two bigger. If she doesn't notice, you're silver. If she likes it, you're golden. Or, lose or break a plug, go to a shop that you know has a limited selection, find one you like that's the size you're shooting for. "Babe, I looked, but the only ones I liked were this size that's bigger."