Our title comes directly from the story itself. Though this passage doesn't appear until almost halfway through the novel, eventually we find out that this—"a little less girl"—is the super mean thing Jake said about Amy. These are the words that everyone in town believes drove her to start crash-dieting and then commit suicide. Amy relays the story in her diary here:
Jane Austen must have died of a broken heart when she realized romantic heroes like those in her stories do not exist in real life. And now, I know that too. You open up their shiny, beautiful packaging and all you find is a black heart. He "prefers a little less girl." Well, I've got news for you, Jake West. I prefer a lot more man. (18.11)
Snap, girlfriend. We think you're too good for him, too.
Basically, this little quote is kind of the catalyst for everything in the story. Because Jake made this comment about Amy, her romantic balloon got majorly popped, and when she died the whole town easily cast Jake as the bad guy in her tragic life story. Jake thinks about this moment a lot once Amy is gone. While he doesn't totally blame himself for her death, he also understands there was something pretty cruel about what he said.
Luckily for Amy, she just shook it off. And in this way, calling the book A Little Less Girl does something clever—in showing Amy's ability to rise above Jake's stupid comment, we realize just how much girl she really was.