How can a tiny bullet have the same momentum as a semi truck?

I don't know if it does, but assuming it does, it's because the bullet is travelling so fast. I believe momentum is figured by multiplying the mass of the object by the speed. A truck is much larger, but moving much slower, so it's possible the figures are close. However, the end result would be different. The effect of a bullet hitting a person is MUCH different than the effect of a truck hitting a person. Contrary to popular myth, bullets don't knock people down (and yes, I know that for a fact, because I have seen people get shot), and they certainly don't propel the persons body anywhere, however, if you got hit by a 10,000 pound truck going 70 MPH, I'm quite sure that would propel your body some distance.