IGCSE's are way higher statistically speaking. This is because you are competing against international students (Chinese, Indian etc) AND against Britians top "private school children" who neccessarily don't want to do bad since they are paying so much money.
As a result, content has to be suited for foreign students as well who may in all respects "require more challenging" material.
Having done both IGCSE's and GCSE's I can safely say IGCSE content goes into further depth in contrast to the new GCSE's and was "easier" to get higher grades.