I don't know why but the larger 1 Kg to 2 Kg packs with course grains don't appear to be as good as the small 500 g tubs that are little more expensive that come from the pharmacy. For example, like the following:
Realistically it should make a great deal of difference because even if the content of magnesium sulphate is 75% this can be compensated for by taking more. Interesting that you should raise this question.