Nobody is surprised that a higher weight produces a higher torque. If fact it is normal that a higher diameter produces more torque. For a torque at lower speed the iron losses are not so important. In fact the limit is rather the continuous force per surface air gap for a given copper loss. Some benchmark is a continous force of 20kN/m^2 air gap using slot depth of 25mm and a copper loss of 10kW/m^2 air gap surface.

In practice only permanent magnet motors get up to this limit, as exciting windings or rotor conductor losses also add to the "copper loss" per surface. Outer rotor PM motors have a higher air gap diameter and will produce more torque for a given outer diameter and also motor weight.

Thank you Mr Alex Van den Bossche 

Typical PM motor torque densities are between 60 and 100 kNm/m^3 of rotor volume which translates to 30...50 kN / m^2 air gap force density. The limiting factors are current density in the stator slots and the properties of the PM material.

Thank you  Mr Jouni Ikaheimo

D.C series or compound motor could be considered as high torque motor, when speed the back EMF is reduced....