Why don't I have neck/chin definition? (Photo)

Thank you for adding your photo to your question. It appears that you have two issues: 1. Small chin and 2. Excess fat under your chin.  The traditional treatment option would be laser liposuction and chin enhancement with either chin implant or fat grafting. 

Thanks for sending  in your question.  You would potentially benefit from a chin implant combined with neck liposuction.  This would enhance your neck contour and improve chin projection.  Non surgical options would include Kybella, Coolsculpting and chin filler. 

Chin/neck definition is something that is influenced by genetics, bone structure and soft tissue. Weight loss and gain can change the amount of fat in the surrounding areas and affect the appearance also. Depending on your x-rays, sometimes bone surgery can move around the maxilla or mandible to get an improvement if needed. Surgical options also include chin implants and liposuction for the neck.  
However, in my practice a lot of patients can be treated not surgically with just filler injections like Juvederm. This avoids surgery and usually the patients are very happy. These procedures can be done safely in the office and the results are often very satisfying. An in-office private consultation and examination with a board-certified plastic surgeon would be the best way to fully  address your specific concerns and determine the right treatment plan.
I hope this helps answer your question.  If it does, please take a moment to up vote this answer on the “respond” button on the top right.