Can I use fillers to fix facial asymmetry? (Photos)

I think one of the issues is enlarged masseters which could be treatment using a neuromodulator like Botox or Dysport. It appears that the right masseter is larger then the left and so I would use a higher dose of Botox or Dysport on the right (larger) side. 

He there!
To answer your question there isnt necessarily a cause per se, all of us will have some asymmetry to some extent. Other factors which may contribute bay include sleeping on one side, or getting more UV light on one side from driving.
I dont see that your right side is sagging too much, but your masseters are quite large, so botox to these areas may help somewhat. Fillers in the cheeks can also help to attempt to balance out asymmetry in some people but results can sometimes be subtle.