Most asteroids are very small; but because they are small, they don't have much mass -- all the mass is in the big ones.
Ceres is about 600 miles (1000 km) in diameter
Several dozen asteroids a few hundred miles in diameter
Hundreds that are tens of miles in diameter
Thousands that are a few miles in diameter
Tens of thousands that are a significant fraction of a mile in diameter
Hundreds of thousands that are a small fraction of a mile in diameter
Millions of still smaller pieces of rock and metal in the ‘asteroid belt’ and environs.
Ceres has a certain mass.
An asteroid 60 miles across is 1/10th the size of Ceres, and 1/1000th the volume of Ceres, and if made of similar materials, 1/1000th the mass of Ceres. Therefore, you would need hundreds of asteroids of that size range to make a significant mass. The actual number is substantially less, so such asteroids only make up about 10 to 15% of the total mass.
An asteroid 6 miles in diameter has 1/100th the diameter, and 1/millionth of the volume and mass of Ceres, so you would need hundreds of thousands of them to make a substantial mass, but there are only a few thousands in that size range, so only about 1/10th% of the mass of the asteroids in that size range.
Ceres has half the mass of the entire asteroid belt.
The asteroids a few hundreds of miles, to many dozens of miles in size, but smaller than Ceres, have about 80% of the rest of the mass.
Asteroids a few tens of miles have most of the rest of that mass, and asteroids only a few miles in diameter or smaller make up only 1/10th% of the asteroid belt mass.
Earth Impact Effects / Probabilities
Because the smaller asteroids are far more numerous, and have far more varied orbits (many are pieces broken off of bigger ones by collisions), they can run into us more often. If a middling small one, five to ten miles in diameter, ran into us, things might be very tough locally, and fairly tough globally for a while, but it wouldn't be a big deal for life on the Earth, in the long run. In fact, asteroids that size run into us every few tens of millions of years, so if it was a big deal, we might not be hear to worry about it. On the other hand, if a really big one (several tens of miles across or larger) ran into the Earth, all life on Earth would be destroyed. Fortunately, since these larger asteroids are much less common, we probably haven't run into one in over four billion years, and probably won't run into another one anytime soon, either.