$$p = mv$$
So, if a bullet has a very high velocity, it can have the same momentum as a truck that has a much larger mass but a lower velocity.
For example, a 10-gram bullet traveling at a speed of 1,000 meters per second would have the same momentum as a 10,000-kilogram truck traveling at a speed of 1 meter per second.
While the bullet has a much smaller mass than the truck, its extremely high velocity compensates for this difference, resulting in equal momentum.