IGCSE is significantly harder, owing to the greater complexity and depth of syllabus. The grade boundaries are not adjusted such that it is equally easy to obtain an A* in both exams. The "I" in IGCSE stands for international, it is an exam taken throughout many countries across the globe and, subsequently, has no relation to the "British" GCSE exam. The exam boards are completely different and make no attempt to align themselves in terms of how hard it is to achieve an A* in each. In fact, IGCSE marks are moderated down more than "standard" GCSE marks are as, generally speaking, students who take the IGCSEs are more likely to obtain top marks*.
I have taught both syllabi and now work on the IGCSE exam board, working towards distribution throughout the UK.
*students who sit IGCSEs in the UK are generally from leading independent schools and, internationally, the exam is very popular in leading schools in Hong Kong and China.