The short answer is NO, you can’t.
Women’s breasts are made of breast tissue and fat. You can manipulate the fat your body carries, but not the breast tissue unfortunately. That’s why plastic surgeons thrive!
Naturally, many women who are unhappy with their cleavage cannot afford plastic surgery and they look for a different way. Manipulating the fat tissue in your breasts works, they get fuller and rounder, but the problem with that is that you gain fat all over. Unfortunately fat spot enhancement is not a thing, just like fat spot reduction (you can’t drop belly fat alone, if you do lose body fat, you lose it all over, the same applies the other way round).
‘But what if there’s another way? Can chest exercises help? I read it somewhere in a fitness for women magazine!’
I’m sorry to disappoint, but it’s impossible to increase the size of your breasts that way!
The chest exercises work on the chest muscles (pectorals), which are underneath the breasts themselves, to reiterate, the exercises do not target the breast tissue, but the muscle underneath it. It will require a lot of heavy lifting and eating in surplus to notice any muscle growth in that area, just like with any other muscle. But because we’re women, we won’t experience a huge increase in size, especially in the upper body. So you will only get a slight lift and separation, that may enhance the look of your cleavage, by an illusion of a deeper separation not by the increase in size. Your diet also plays a role. If you’re exercising and are eating in calorie deficit, you’ll be losing fat, hence boobs.