I'm not too materialistic but I know if I got a bigger rock I would be more impressed than if it was a smaller one. But I am more for the overall appearance and the proportions of the ring. I prefer a smaller but higher quality over a larger and lower quality, and not to mention bigger one gets in the way of everything.

I don't think wanting a bigger diamond makes a girl not worth marrying, but the fact that she is willing to say something you took time, effort and money picking out is not enough should be a huge red flag to get a new girl.