I feel as if my mandible is extremely weak. Would a sliding genioplasty make the jaw look bigger or only the chin? (Photo)

 Thank you for your pictures. I read that your proportions to your face could be improved. I think that you should start with a good dental-facial analysis buy an orthodontist. In my opinion, you would need of at least a lower jaw surgery possibly chin surgery and a rhinoplasty to balance out your facial profile. 

 A rhinoplasty procedure can address narrowing the tip and bridge  and de- projecting the nose, but it's important to have realistic expectations with very thick skin. A chin implant  can be inserted through a submental approach to give added projection to the chin  to help significantly with facial balance and proportions, especially from  the side profile. For many examples and more information, please see link and the video below

The combination of a genioplasty and jawline augmentation will achieve your goals. Consult a Maxillofacial surgeon.