A few factors in play:
A lot of breast development is weight-related, so heavier girls will have more developed breasts even before HRT or female puberty, regardless of their birth sex.
The body's fat cells store some estrogen, and more of it is released during weight loss, so heavier male bodies naturally have more estrogen and estrogen effects on development especially when they lose a lot of weight.
Some commonly ingested substances and some non-HRT medications promote gynecomastia. For example, hops in beer. Hence, 'beer boobs'. I won't list all of the medications. There are environmental chemicals that do the same thing.
The hormone cycle is a complete circle, so excessive androgens will be converted into estrogen. The men in my family have very high testosterone, which makes them have a very high estrogen level for males. Mine was higher than my wife's before I started HRT.
Both males and females produce some testosterone and some estrogen for health. The testes/ovaries are not the only glands that produce sex hormones, though they produce the lion's/lioness' share.
Some of us are intersex as well as transgender, and may have naturally high estrogen production.
Rarely, tumors in other parts of the body can produce testosterone or estrogen. These can be benign or cancerous.
Some people are chimeras (having the tissues or genes of more than one individual), theoretically due to the natural processes of twin absorption or mother-child cell transfer in the womb. If one has tissues of both a male and a female individual, this can create complicated mixtures of physical traits including hormone production and hormone responsiveness or insensitivity.
That should about cover it!
