The ULN2003 is a Darlington array correct? If that's the case then some heating should be expected, however ericgibbs is right. 700mA does exceed the maximum current of 500mA. Connect 2 in parallel and you should be okay.
ULN2803 would be a better choice for using with a small stepper motor. There is 8 darlingtons instead of 7 for the ULN2003. But at 700 mA, even paralelled, it is asking too much from this chip. Sooner or later, it will smoke specially if you try to run the motor full step for better torque, that being 2 phases energized at the same time. ULN2803 is pretty good for small motors rated at 150-200 mA per phase.