Edexcel GCSE and IGCSE Maths are different exams, rather than one being harder than the other. One big practical difference is that GCSE has a calculator paper and a non-calculator paper. In IGCSE both papers allow calculators to be used (being able to use a calculator doesn't necessarily make a paper easier). There are also some differences in the topics covered. The most obvious difference is that IGCSE includes some introductory calculus, which is not in GCSE. IGCSE also includes some minor topics in geometry, vectors, and series that aren't in GCSE. But then GCSE includes some topics in numbers, sequences, functions, geometry, and statistics that aren't in IGCSE.
There are a number of differences between the A and B specifications: IGCSE Maths A is more like GCSE, in that it is offered at both Higher and Foundation tiers, and is meant to be applicable to the full range of mathematics students. IGCSE Maths B is meant for high-achieving mathematics students who are likely to continue to study maths at A Level or beyond. Therefore IGCSE Maths B is only offered at the Higher tier. Maths A has two 2-hour papers. Maths B has a 1-and-a-half-hour paper and a 2-and-a-half-hour paper. The longer Maths B paper includes a number of multi-part high-mark questions (10-16 marks), that are meant in their style to be more similar to A Level questions. Maths B has some topics that aren't in Maths A (for example matrices, along with algebraic division and the factor theorem). But Maths A Higher Tier has some parts of topics that aren't in Maths B (topics in statistics and sequences, along with graph transformations of functions).
Please note that the grade boundaries can vary quite a lot depending on the year and the version of the paper! Students shouldn't count on getting a particular number of marks as making them 'guaranteed' to get a particular grade.