Smaller Nose After Excercising at the Gym?

I would not be overly concerned about fluctuations of nasal size with exercising.  Once you are about 1 year post op there should be not further changes.  

After rhinoplasty surgeon there may be some changes and the soft tissues settle and the nose heals.  This is normal and the reason most rhinoplasty surgeons do not recommend revisions until after this time.  Depending how the soft tissue remodels as the fluid shifts your nose may have appear narrower.