That truck is a part of a huge set excavator in real life. In real life, half the height of the wheel is still taller than a 6 foot person. The company that makes those kinds of trucks is called Terex corporation, stock symbo TEX. Here is a link the actual schematics of that huge truck. And yes I love big machines like that :)
http://terex.com/main.php?obj=prod&action=...c712d1cc68a5688
They are so huge because the excavators that extract the dirt is in huge amounts and so that both trucks and excavators have to grow in proportion.
CAAAAAAAAAAASTLEAAAAAAAAAAAAAAAAARGH