The human skeleton of a female body. The weight of human beings do not come all from the flesh and bones. The bones and flesh and fluid etc would constitute only 1/5th of the total weight the rest 4/5th comes from the amount of space inbetween all such matter. The body weight therefore would drop to 20% if the matter such as bone and flesh would be extremely dense, but in such a case body would occupy a thin lamina of space but the resultant reactions inside the body would be destructive and the body would break apart like bricks hitting stones. So the body empty space is optimized and provides a cusion to all sorts of biological and physical reactions inside the body and in result increases the weight (by 80%) and volume (to the extent we see). Next time you are tempted to call someone a cow realize that its not the flesh he/she has. Its the ability of nature to put that flesh into a space for smooth functioning of the biological body which means a 5 times more space in terms of weight and given density of material the empty space occupies so much or so much space. And a s a result we swell.