How big difference is there in torque on a nema 14 compared with a nema 17 stepper if they are driven with the same current (for example 1A)?

There is another variable there. It depends on the number of windings too. Many steppers have multiple versions of the same model with different current and the same torque.

Now all other things being equal, a nema 14 will have less torque with the same current and number of windings just due to the size, since torque is force times distance.