Pakistanis has targeted to achieve replacement level of fertility that is 2.1 births by the end of 2025 (national population policy, 2010). But this target cannot be attained without knowledge of factors which affect family size. The empirical study is devised to investigate the contribution of various socioeconomic, demographical and attitudinal factors which force couple to have more than two children. Complementary log-log Regression Model has been used to identify factors affecting the family size. It is found from Multivariate Analyses that women’s age, husband’s education, women currently not working, lack of consensus between husband and wife on number of children, son preference, high fertility intention, contraceptive knowledge, contraceptive use and child mortality are responsible for big family size (more than two children).