Will injecting Botox into the Buccinator muscle make my face appear slimmer?

Hi rcdivina, thank you for your question! Botox to the buccinator muscle would not give you the slimming effect you would like to achieve. To best achieve this look botox should be placed to the masseter muscles. Multiple treatments may be needed in order to see desired results. 
The buccinator muscles are deep muscles that assist muscles of mastication, smiling, and whistling. Injecting Botox to these muscles will effect your ability to properly carry out those vital functions. 
I've created  a video that reviews treating masseter muscles with Botox to slim the face. I hope you find this helpful in your research! 

 I think what you mean is should you get botox injected into your masseter muscle. If that alone does not slim your face down, you may have to actually get your buccal fat pads removed by a facial plastic surgeon. Good luck, Dr. Downie

You will end up will problems smiling after injection in this area. If you want slimming after having the masseters appropriately injected, then consult a surgeon to consider buccal fat removal. 

Without pictures it is difficult to tell what is making your face chubby. I doubt that it is the buccinator muscle. There is a fat pad in that area called the buccal fat pad and that may be the cause of your chubby appearance. I would talk to a board-certified plastic surgeon about this and they may suggest excision of that buccal fat pad.Good luck! 

You have posted no pictures to indicate why your face is "really chubby". If its fat, then Botox will do nothing. If its Bone, then Botox will do nothing. If if excess facial volume is due to masseter muscle hypertrophy then longterm proper injection of Botox will atrophy the muscle and reduce the volume. Buccinator muscle hypertrophy is not the reason for "chubby" face and should not be injected with Botox as it will cause problems with oral competence.