Weak magnetic field because back EMF is proportional to field intensity.
If you have a parallel wound DC motor with separate rotor and stator connections, you make it go faster by reducing the current in the stator winding.
Likewise, series wound motors have metric shitloads* of torque, because at low RPM, back EMF is low, so the equivalent resistance of the motor is very low, current draw is very high, and stator field is very high. Series-wound motors do not have a constant torque to current ratio, but rather a parabolic curve because the rotor current is multiplied by the field current. (Squaring a negative value also makes a positive number, which is why series motors cannot be ran in reverse: they are also known as 'universal' motors, because they run on AC.)
*A technical term.
Tim