I'm not overweight, but my face looks fat when I smile. What's the most cost effective way I can fix/improve this?

Chubby cheeks are usually genetically determined and resistant to decrease in size with diet and exercise.
Buccal fat removal is the most cost effective plastic surgery procedure that corrects chubby cheeks. By removing the fat that causes the chubby cheeks ( buccal fat) patients could achieve a slimmer V shape more chiseled face. The video bellow helps to understand who could be a candidate for buccal fat removal plastic surgery.

Buccal fat excision is an appropriate surgical procedure for patients that of overly full and wide cheek areas. In common terms, it is good for patients who have chipmunk cheeks. Liposuction can be used to further slim the face. In reviewing your pictures, is difficult to tell if you're a good candidate because your cheek area is slightly full but not overly full. You are a patient that would need and in person consultation for a complete evaluation and explanation of the positives and negatives of these procedures in your situation.
I hope this helps.

Hi kaliebird92,Thank you for your question! I have to disagree with you that your smile looks terrible. I think that you have a very nice smile! If you want to thin your lower face, we can remove the buccal (cheek) fat, perform lower face liposuction, and inject Botox to the masseter muscles of the lower face. Of all these things, the Botox injection to the masseter muscle is probably the most cost effective, although it is not a permanent treatment.Please let us know if you have any other questions or concerns!

Hi @Kaliebird92 
You would benefit with seeing a plastic surgeon to possibly perform masseter botox and bilateral buccal fat pad removal. Overall you have a very attractive face and these two procedures might help get you what you're looking for. 
The ideal candidate has fat in the cheeks that is above the border of the mandible or jawline and towards the front of your face. This is easily treated with buccal fat pad removal. 
 If the prominence or “chubbiness” is towards the back of the jaw (like under the ears), then this is actually an oversized masseter muscle which can easily be treated with Botox which will shrink the muscle. This typically requires 25 Units of Botox on each side.
At this time there is no laser, injection or facial exercise that can help with buccal fat pad loss. Some websites promote facial exercises and cardio workouts that promise to reduce facial fat. Others discuss a low salt, low carb and low alcohol diet. Unfortunately none of these can provide instant and permanent reduction in face fat like a buccal fat pad removal.  
Best of luck,
Dr. Karan Chopra
★★★★★
Minneapolis Plastic Surgeon
Face ⨀ Breast ⨀ Body Expert
Johns Hopkins Trained
Cosmetic Fellowship Trained

It appears that there is increased fullness in the mid cheek area and Buccal fat removal would decrease that and give an improved more heart shaped contour. Prior to doing that it would be necessary to have a photo without smiling to fully evaluate this. 
Buccal fat removal is done through an incision in the mouth. This procedure has relatively short and uneventful recovery.
Liposuction of this area is more problematic since that could result in some nerve problems. 
You should have a consultation with a Board Certified Plastic Surgeon  to discuss this.