Androcur (cyproterone acetate) is a progestogen/progestin, not 'a progesterone'. There is progesterone and then there are progestins. They all have differing effects, for example some of them have androgenic effects such as Levenorgestrel, and others have anti-androgenic effects such as cyproterone acetate.
They are all separate and distinct in their own right with their own side effects and risks. The fact that they are all lumped together and called progesterone by many has lead to confusion such as the DVT risk, which is associated with progestogens such as medroxyprogesterone acetate, but much less with micronised progesterone, such as that in prometrium.
Whichever side of the argument that you are on re breast growth, prometrium is still the one to be on due to the fact it is regarded as better tolerated and, depending on which side of the fence you are on, arguably more effective.