The Canon by Natalie Angier:

The largest egg in the world, and thus the largest cell in the world, is the ostrich egg, which measures about eight by five inches and weighs three pounds with its extracellular shell, two pounds without. (Interestingly, the ostrich egg is also the smallest bird egg relative to the size of its mother, amounting to only 1 percent of the female ostrich's body mass.  The she-birds most deserving of every mother's pity are the kiwis and hummingbirds, which lay eggs that are 25 percent as big as they are - the equivalent of a woman giving birth to a thirty-pound baby.