Wafer defects, which, among other reasons, are caused by random dust occuring between the ultraviolet lamp and the wafer (https://en.wikipedia.org/wiki/Photolithography). For some fixed concentration of dust in environment, the larger the die, the bigger the fraction of dies which have at least one defect (the lower the yield). It also depends on how robust the circuit design is to defects - defects could be detected, faulty circuit portions could be localized and then the circuit could route work around them.