I working on a project and witnessed that there are different stepper motors with same NEMA DD but different weight and different amp rating. what does these different notations with same NEMA signifiy and how does these affect torque?
The NEMA number specifies only the physical size of the motor. Motors with the same size can have different torque / current rating / voltage rating etc.
Generally, motors with the same number of steps which dissipate more power (amp rating multiplied by voltage rating) will have more torque since you're putting more energy into the rotor to make it travel the same angular distance.
All of these parameters should be noted in the motor's datasheet, so when you need a motor with a specific torque, look for its torque rating.