Rate of metabolism is low  in animals of more body weight . so, rate of heart beat is slow in the organisms  of more body weight.

eg: Blue whale - body weight : 1,50,000 kg. - heart beat : 7 bits / min. - weight of the heart : 750 kg.

Weight of the body is directly proportional to body weight.

weight of  the heart is inversely proportional to heart beat.

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Basal metabolic rate is low for the organism with more weight ie the metabolism is carried out slowly in organisms with higher rate,so the rate of heart pumping in order to carry out any metabolism is also low.hence the rate of heart beat is low