Unsurprisingly there are lots of thoughts regarding transdermal Estrogen products applied directly to the breasts on the internet . One is that it can increase the risk of breast cancer. The minimal research I did suggested this isn't really a big increase in risk, but bare in mind manufacturers don't recommend application here. What I did find is that its apparently not optimal due to skin thickness and fat tissue storing and delaying release which can reduce efficiency.
In reality if E levels are in the correct range then location of application, or higher / lower levels don't affect breast growth. Most important is suppression of T followed by being in range with E.
I'd recommend just applying on the arms or thighs as most instructions suggest.