If I recall, there's actually very little material in this region. It's not like you see in the films with loads of asteroids shoulder to shoulder. I suspect with most of them, they're so far apart, one can't even see another.
It depends on how you look at it.
In terms of mass, yes the asteroid belt is quite small compared to even our moon, let alone the earth.
But unlike the earth and any other planet, all mass is accessible because it's fragmented into small parts.
On the earth we can only really mine the thin shell of rocky surface, and we can't even effectively mine where there is ocean.
So practically speaking, there may be many many times the amount of resources up for grabs in an asteroid belt than there is on earth.