What is the largest lake in the world? Compare this to 'the biggest lake'. To my mind, the largest is the one with the greatest surface area, the biggest may have a smaller surface area but be deeper and therefore contain more water...and be 'bigger'. Of course, one could just as well assign the other way..but there IS a distinction. I think.
"Largest single" is not incorrect (think "this is the largest single cell"), it just has a different meaning. But I cannot at the moment conceive of a sentence with "the largest single contributor to". Therefore, in your examples, "the largest single contributor to" seems incorrect, although the full context could prove me wrong.
What about the second pie chart? In the second pie chart the largest portion is less than 50%. Can I still consider the group "c" as majority in it? Based on the Cambridge Dictionary it seems ambiguous: (1) the larger number or part of something, (2) more than half of a total number or amount.