It's about one-eight-hundred-thousandth as heavy as a Grizzly Bear.
In other words, 645.5 miligrams is 0.00000124 times the weight of a Grizzly Bear, and the weight of a Grizzly Bear is 806,000 times that amount.(a.k.a. brown bear, a.k.a. silver bear, Ursus arctos horribilis) (Adult, male, coastal area population average)
An adult, male grizzly bear from a coastal area (where food is plentiful) can weigh up to 521,000,000 miligrams. In spite of their tremendous size, grizzly bears can run at speeds of up to 55 kph (34 mph).
It's about 0.0000010 times as heavy as a Cow.
In other words, 645.5 miligrams is 0.00000095 times the weight of a Cow, and the weight of a Cow is 1,100,000 times that amount.(for Holstein, a.k.a. Holstein Friesian, a.k.a. Friesian, Bos primigenius) (adult, heifer)
A mature Holstein cow (the well-known black-and-white, spotted breed of cow) typically weighs 680,000,000 miligrams. President William Howard Taft (1909 - 1913) kept a Holstein cow named Pauline Wayne on the White House grounds — a gift from a Wisconsin senator and the source of the First Family's milk.
It's about 0.0000004 times as heavy as a Car.
In other words, 645.5 miligrams is 0.0000003907 times the weight of a Car, and the weight of a Car is 2,560,000 times that amount.(for 2009 Ford Taurus; automatic transmission; curb weight)
A 2009 Ford Taurus weighs 1,652,000,000 miligrams. The first mass-produced car, the Curved Dash Oldsmobile — introduced in 1901 — weighed just 386,000,000 miligrams.
It's about 0.0000003 times as heavy as a Rhinoceros.
In other words, 645.5 miligrams is 0.00000031 times the weight of a Rhinoceros, and the weight of a Rhinoceros is 3,200,000 times that amount.(for Black rhinoceros, Diceros bicornis) (Adult, male)
An adult black rhinoceros weighs between 2,100,000,000 miligrams. Rhinoceros are extremely aggressive — nearly one of every two males will die as a result of fighting with other rhinoceros.
It's about 0.0000003 times as heavy as a Giraffe.
In other words, 645.5 miligrams is 0.00000026 times the weight of a Giraffe, and the weight of a Giraffe is 3,800,000 times that amount.(Giraffa camelopardalis reticulata) (adult, male)
An adult, male giraffe weighs an average of 2,500,000,000 miligrams. Reaching 5.5 m (18 ft) in height by adulthood, giraffe calves can grow as much as 3 cm (1 in) per day.
It's about 0.0000003 times as heavy as The Tongue of a Blue Whale.
In other words, 645.5 miligrams is 0.00000024 times the weight of The Tongue of a Blue Whale, and the weight of The Tongue of a Blue Whale is 4,200,000 times that amount.(Balaenoptera musculus) (adult)
The tongue of a Blue Whale can weigh up to 2,700,000,000 miligrams. Despite their tremendous size, blue whales feed almost exclusively on tiny crustaceans called krill and cannot swallow anything much larger than a beach ball.
It's about 0.0000002 times as heavy as a Hippopotamus.
In other words, 645.5 miligrams is 0.000000172 times the weight of a Hippopotamus, and the weight of a Hippopotamus is 5,810,000 times that amount.(Hippopotamus amphibius) (adult, male)
The average weight of an adult male hippopotamus ranges from 3,750,000,000 miligrams. Appearances to the contrary, adult hippos cannot swim or float in water; they actually move by leaping and walking across lake bottoms at about 8 kph (5 mph).