Can binding or T make your breasts smaller? I apologize if this triggers anyone.
Short answer, no. *Sometimes* the fat in your chest can redistribute elsewhere, but probably not a lot and probably not in most cases.
Binding will make your chest appear flatter.
Testosterone can make exercising work better and can do awesome stuff like losing fat and gaining muscle which can help with the chest area. Or, like Ty Turner who is all muscly and stuff and his chest looks natural