What reasons could one side of my face look swollen/bigger and the other defined? (photo)

Most people's faces are not completely symmetric and I would say that degree of asymmetry in your face is just natural variation.  If you feel the issue is worsening, you should be evaluated medically.  Most likely, there is not a medical condition causing the difference between sides.  You will notice the difference more than anyone else because you see your face more than anyone else.  A casual observer would most likely notice no difference.    

I suggest a formal consultation with a cosmetic dermatologist who specializes in non invasive approaches as it is likely this is what would do best for you.  Dr. Emer.