When you are rating motors or mechanical systems, you have two ratings to think about.
One is the speed which is obvious - the other is the torque which is less obvious.

If you wish to lift to objects at the same speed, the energy needed to lift a heavier one will obviously be greater, and this will need to go through what ever transmission system you have, as well as be generated by your actuators. So if you increase the force needed to move an actuator - you will increase the torque.

Going over the rated torque, or underestimating the torque for a system will result in stalled motors, snapped axles, worn teeth on gears and other nasty breakages.

Note that a stalled motor becomes effectively a short circuit - which is extremely bad and may cause batteries or capacitors to heat up.

If you wish to increase the torque a system can handle - you need to increase the axle diameters - and reduce their length. You can also double up the axles/gears for a particular transmission.
You can use Helical Gears to increase capacity - or simlpy larger/wider gears.

Larger motors will deliver greater torque - although the diameter of their axle is a big factor in this. The more torque is required of a motor, the greater its current draw will be- so be aware that it could put a huge current dip in the system.

Too much friction in a system, or bad fitting teeth on gears can cause larger torque - as the system is now having to carry that extra load as well as the actual work done.

http://www.solarbotics.net/bftgu/tutorials_mech_torque.html