Hello,
For MTFs, does taking estrogen cause breast growth even if you take a small dose?  What actually grows; the breast tissue or the fat?

Yes - estrogen is what causes breasts in cis girls too, honestly. And it's both; breasts are partly fat and partly ducts and whatnot, and estrogen will cause the development of both fully mature breast tissue and the depositing of fat to round them out.

What effect it will have at a given dose varies by individual. (I had good results at a very low dose, but some bodies need more and some need less.)

Thank you for your answer. I am not MTF but I am desperately trying to find out if there will be any growth (breast tissue OR fat) on my chest after I have the breasts removed with top surgery...  because my body will still produce estrogen (I am not planning on taking T anymore).

From what I understand once the 'receptors' are removed they won't grow back.
Like people who get liposuction , once they start putting weight back on it grows lumpy in lines ,not all the fat is removed. Just particular areas.
Ask a plastic surgeon or endo.
Or even better post the same in detransition section maybe others who have will ,or are , better informed through experience

Oh!! I'm sorry, that's a completely different question. Once the glandular tissue and breast buds that support breast growth are removed, they will NOT regrow. It's like voice changing or facial hair - hormones can trigger a one-time-only development phase that will never happen again. Once breasts are developed and then removed, they're gone.

You may, however, see some fat redeposit in the area. Cis guys get moobs too if they're overweight.

That's fine, I should have worded it better. I hope you're right though, I still haven't received a reply from the doctor so that's why I wanted to know. I will keep my weight stable for sure.  Another thing I am worried about is taking a progestin only mini pill to cease periods.  Will progestin cause anything like estrogen does?

My endo and I have chatted about this. Apparently there are two glands in this area? Or at least two that he described. Once estrogen begins to impact these glands, they change shape and begin growing. There is apparently one at the base of the breast and one closer to the nipple area. Removal of these will most likely prevent further development.

Perhaps someone a bit more knowledgeable of anatomy can comment? I'm relaying second hand a brief explanation from my endo so it's likely to be wrong and/or incomplete.