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It is true that the finer the grain, the higher the strength of resultant concrete. But this is only half the story. Finer grains (like those of Silica fume) help in filling up voids of concrete which are otherwise filled by air. We need higher sizes of aggregate also to get strength. The aggregates should be well graded and larger the maximum size of aggregate the smaller the quantity of cement to get the same strength. Thus for the same cement quantity, maximm size of aggregate will give a higher strength provided the aggregate is well graded.

Thus to get maximum strength for a given quantity of cement we should have finer particles as well as higher sized particles and all aggregate should be well graded.

I trust this answers your question.

Sincerely,

Dr. Sharma