Cormorants range in size from as little as 45 cm ( 18 in ) and 340 g ( 12 oz ) to a maximum size 100 cm ( 40 in ) and 5 kg ( 11 lbs ) . Cormorants and shags are medium-to-large seabirds . They range in size from the Pygmy Cormorant ( Phalacrocorax pygmaeus ) , at as little as 45 cm ( 18 in ) and 340 g ( 12 oz ) , to the Flightless Cormorant ( Phalacrocorax harrisi ) , at a maximum size 100 cm ( 40 in ) and 5 kg ( 11 lb ) .