Why does the T Rex have tiny arms?

The (relatively) tiny arms of the Tyrannosaurus Rex were used to hold food in position next to the mouth, to be eaten. It was not necessary for the arms to be larger since the creature would just bend down to reach things if necessary, rather than reaching down with its arms. It was strictly bipedal, which is to say, it only walked on its legs, so the arms did not have to serve any other function.