Current lightest geared hub motor is a signwave AKM75sx at 1.44kg for 100mm forks, £81.
Bafang's lightest is 1.7kg but nearly twice the price, £150.
The smallest lightest rear motor is a Keyde 100/s100 with an internal controller, the Cst is for 135mm D/O's and comes in at 2kg, 35nm torque...