Rigidbody.mass

The mass of the rigidbody.

You should strive to keep mass close to 0.1 and never more than 10. Large masses make physics simulation unstable.

Higher mass objects push lower mass objects more when colliding. Think of a big truck, hitting a small car.

A common mistake is to assume that heavy objects fall faster than light ones. This is not true as the speed is dependent on gravity and drag.

JavaScripts

rigidbody.mass = 0.5;

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void Awake() {
rigidbody.mass = 0.5F;
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def Awake():
rigidbody.mass = 0.5F