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According the paper 'Information propagation in the bitcoin network'. by Decker and Wattenhofer.
For sizes larger than 20kB the cost can be said to be constant, whereas for small sizes there is a considerable overhead. Actually, I didn't get how small blocks take more time than messages whose size is greater than 20KB ?
How delay taken by blocks with size greater than 20KB is almost constant? As shown in image below:
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