Depending on your age, and your genetics you inherited, the impact can be minimal.
Starting HRT in your 20s will have a better impact than if you start in your 40s or 50s.
My mom was no bigger than a B cup, which meant my six years of HRT really only got me to a B.
I suffered no weight gain from HRT, but some do.
I get met with a lot of resistance to this, but I never would have self medicated. I don't care what the Internet says, or what you think you need....only an endo and/or doctor can tell you.
I see my endo twice a year....she's amazing. She doesn't just monitor my estrogen (I'm off progesterone and spiro for good), but my levels of B12, D, A....my calcium intake etc.
Final point, IMO, hormones do not make you a woman. What if, for some reason, you have a bad liver? Does that mean because you can't start HRT that you can't transition? No way. I would have transitioned regardless of hormones.