Why giraffes will become short again?

Elephants, rhinos, kangaroos, camels, all mammals evolved at the first time by small bulk animals. Mammals have managed to overcome gravity by the aid of 4-chambered hearts and strong blood circulatory systems and making their bulks larger. But during the time a gradual increasing of gravity occurred, mammals failed against greater gravity and had to minimize their bulks. The process of minimizing the body size has continued up to now and will continue in the future Mammals will become gradually smaller.