Hi there Samantha and thanks so much for posting this question.
Yes I am very happy that you are posting this question since every time that i get asked the same question i always respond that you can have firm chest muscles without the need of getting surgery or feeling you are getting bulky and believe me saggy breasts need a strong muscle for support. So here you go my recommendation to you!
The major muscle in the chest is the pectoralis major, which powers movement in the shoulder and keeps your arms connected to your body.
I am proposing three different exercises to help you develop strong muscles in your chest to have firmer and more toned chest muscles:
Leg Curl with Push up
Leg bridge with chest fly
Plank with Push up
Thank you so much for that information.