Can a chin implant make for a slimmer face?

Without an exam your question cannot be answered.  You will need to see an Oral/Maxillofacial surgeon to see if othognathic surgery, genioplasty or using an implant is best for your specific case. It should be noted that patients with severely weak jaws or abnormally functioning jaws may be better treated with other facial surgery (orthognathic surgery), although a chin augmentation may be an option at a later time. These patients should seek the advice of a plastic surgeon to determine which course is right for them. Here is some information to help you make this decision: Candidates for Chin Augmentation - Patients whose chin profiles are weak but have normally functioning jaws are candidates for Chin Augmentation. Many of these candidates have used injectable fillers in the past to strengthen the protrusion of their chins, but are looking for a permanent solution. If you are one of these patients, then you are a likely candidate for chin augmentation
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The proper style and size selection of a chin implant can help make the face look thinner. But the key is to have one with a heart-shaope design that keeps the chin narrow but also adds a little vertical length as well.

  Chin implants are placed for patients who have a weak and recessive chin profile to augment the chin forward to help with facial balance and proportions.  Chin implants are not placed for patients to try to alter the shape of the face.  For more information about chin implants and many before and after examples, please see the link and  the video below

You need to submit  couple of good photos before you can expect to  get any sort of useful recommendation.

Hi, I have performed many facial shaping procedures using dermal fillers, facial implants (cheek, chin), liposuction and/or facelifts for over 30 years.  Non smiling photos of your face from the front and side would help in the evaluation. 
 Following my the facial beauty principles that I've developed over the past 30 years I have found that women look the most feminine, youthful and attractive with heart shaped faces.  Heart shaped faces have cheeks that are full and round in the front.  Cheek augmentation with a dermal filler or using cheek implants for a permanent enhancement will create full, round cheeks that will feminize the entire face.  If your face is round, oval or rectangular, cheek augmentation can create the more feminine heart shaped face.  If excess fat exists in the face and or neck, liposuction would be combined to further shape the cheeks, face and neck.
 If your profile appears out of balance, it could certainly be due to a weak chin.  I have performed many Chin Augmentations using chin implants for over 30 years and have found that a weak chin creates an imbalance making the nose appear larger, the mid face top heavy and the lower face look short that de-emphasizes the lips and allows early formation of a double chin.  Chin augmentation using a silastic chin implant will add projection to the chin creating harmony and balance to the lower face. 
 I place the silastic chin implant, through a small curved incision under the chin (also allows excess skin removal) to be very safe, quick, highly effective and far less invasive than a sliding genioplasty.  I perform chin implant surgery in 30 minutes or less, often using a local anesthetic alone.  In my opinion, you are a good candidate for chin implant surgery.
Hope this helps