I am little bit confused with dual statements which I read in the literatures and books. Some says that Non-inferiority studies requires more sample size than superiority study and other states vice versa. Can anybody guide me what is correct here?

Also as per some experts the case where the treatment of interest appears slightly less effective than the control treatment the sample size for a superiority study is significantly smaller than that of the equivalent and non-inferiority study. How it is possible if test is less effective how come we prove its superiority over reference?