Price is proportional to die size. Processing one wafer is a fixed price, so the more chips you can fit on one wafer, the cheaper each individual chip.

Additionally, the number of defects is proportional to the area. The bigger your chip, the more chips you will have to throw away because of defects. E.g. say you have one defect per wafer on average, if your chip takes up the whole wafer, you will have no good chips. If you can fit 100 chips on one wafer, you will have 99 good ones and one bad one.