The pupil of the right eye of my girlfriend (18, contact lenses)is bigger than her left one. Can pupil sizes be different normally or is it a pathology?

Yes, & maybe. Some people are born with different sized pupils & it doesn't mean anything. If she's only recently developed them, it could be a sign of neurological problems, though. I recommend looking at old pictures- if they were different when she was little, like they are now, there's nothing to worry about.
Anisocoria. This could be a normal variant. Look at old pictures. If seen on old pictures then is most probable normal we need to figure out which pupil is not working (smaller or the larger one ) also figure out if it is pathological process. See an opthalmologist or neurophthalmologist.