What you're interested in is the power they can handle. If you look at Neu's specs:
1500 motor specs
you can see that the:
1512 is rated to 2000 Watts
1515 is rated to 2500 Watts
1521 is rated to 3500 Watts
etc.
It's not the kv that matters for choosing a motor for a specific size vehicle, it's the physical size of the motor and the power it can handle. Then you choose kv for the particular gearing/battery voltage/etc. of your setup.