I’m not overweight, but when I smile my face/cheeks looks deformed/fat. What is the cause and what can I do to fix this? (Photo)

Asian background patients that  are not overweight but have a fat face specially during smiling, usually have enlarged buccal fat pads.
Enlarged buccal fat pad is the cause of chubby cheeks. They usually run in families. They are also commonly resistant to decrease in size with diet and exercise.
Buccal fat removal is the plastic surgery procedure that corrects the chubby cheeks. I routinely perform it under local anesthesia in the office setting.
The video below helps to understand who could be a candidate for buccal fat removal. 

Thanks for your question and Pictures
The buccal fat pad is a rounded mass of fat in the middle of your cheek. It’s located between facial muscles, in the hollow area beneath your cheekbone. The size of your buccal fat pads affects your face shape.
Everyone has buccal fat pads. However, the size of buccal fat pads can vary greatly.
If you have larger buccal fat pads, you might feel like your face is too round or full.
You can be a candidate for a Buccal Pad removal surgery.
Best of Luck

People have different sized cheeks and that is made up of your buccal fat, your inherent subcutaneous cheek fat and your cheek bones. The buccal fat is overall a small amount of that, enough that even when you remove it, it won't make a major difference to your smiling view. That being said it is the only thing you can do so sometimes, even though it doesn't help an amazing amount, it can still be worthwhile.