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This is about the degrees of comparison. It's easier to begin with English. In English, the adjective "great" denotes the "positive" degree, "greater" denotes the "comparative" degree, and "greatest" denotes the "superlative" degree.
Here in Arabic, "Kabir" is the "positive" adjective, "akbar+preposition" makes a comparative adjective, and "akbar+noun" makes a superlative construction.