The largest animal is a blue whale, and the largest land animal is an African bush elephant. The average mass of a blue wheel is more than 22 times that of an average African bush elephant. The maximum mass of a blue wheel is more than 15 times that of the largest African bush elephant. Link.
Why can blue whales be so many times larger than an African bush elephant? Galileo Galilei had an answer in Two New Sciences (TNS). TNS is written as a dialogue in which Salviati played the part of Galileo’s own spokesman.
Salviati: "Well, the ability of fish to stay motionless in water is a convincing argument that the composition of their corporeal bulk is equal to water in specific gravity. So if some parts heavier than water are found in them, there must necessarily be an equivalent amount less heavy in order for equilibrium to hold. So if the bones are heavier [than water], it must be that the flesh, or some other material present, is lighter, and that these offset with their lightness the weight of the bones. Thus, what happens in aquatic animals is the opposite of the case with terrestrial animals; namely, that in the latter, it is the task of the skeleton to sustain its own weight and that of the flesh, while in the former, the flesh supports its own weight and that of the bones. And there the marvel ceases that there can be very vast animals in the water, but not on the earth, that is to say, in the air." Link.
A large part of blue whales is blubber. This page cites 50 tons, or about 45.5 tonnes. If that were from an average sized adult blue whale, it would be about 41% of the body weight. The density of blubber is very near that of seawater (65 pounds per cubic foot).