To your own perception, the changes will probably not seem that big. Just about every single trans-woman that I've ever met has said that they really didn't think they've changed that much. The thing is, though, I don't think a lot of us realize just how big of a difference these small changes can make.
For me, the biggest difference has been the skin texture. My skin is now starting to take on that creamy-white feminine glow to it, which does wonders for softening the face, and softening the body overall, into a more feminine appearance. This is pretty much universal for everyone on HRT. Almost everyone gets much softer, clearer skin. Also, for me, HRT pretty much knocked all of my body hair clean off, so that's a huge plus as well.
Muscle atrophy has indeed happened, but so far it's been more of a "feel" thing than anything. People do tell me that I'm looking skinnier, but I haven't noticed the visual aspect so much as the "feel" of it. My arms feel lighter, and freer, and I've really started moving in a more feminine way naturally because of this change in muscle structure. My voice has gotten a bit softer, and I move more gracefully rather than forcefully. This to me is the real change, is in the way you move, not so much the way you look. But it also has helped in softening the appearance of my body too. Not so much shrinking, but softening. (This is probably because I'm overweight, so most of my body's bulk is fat, and thus has not shrunk along with the muscles.)
I have gotten some boobage (up to a full B from a high A when I started,) and there has been some fat redistribution, mainly that I've put on fat in the thighs/butt, but it is very SLOOOOOOOW. Some who are predisposed for it will blossom quickly, but it's uncommon to get miracles here. There's a reason why most cis-girls really don't start looking full and womanly until they're like 17-18 or so. It can take a LONG time to start really putting on curves.
The thing is, though, while all of these changes are subtle, the combination of every single one of them, combined with female hair and female clothes, is almost always more than enough to tip the scales to the point where just about everyone will be seeing a girl when they look at you. So far, I've only ever met one person under the age of 35 who didn't get sufficient results from HRT in order to start passing. And those who started anywhere from their teens to their mid-20s, you'll usually be hard-pressed to tell that they were ever male unless you're specifically looking for common transgender features. So in short, HRT can be a miracle worker in certain people, especially if you're younger, some people get almost nothing from it, but most will probably see sufficient results in order to pass as female. I've seen very few who pass as female pre-HRT, but most pass after it. So while the visual difference may not seem like much, it makes a HUGE difference in terms of tipping the scales from male to female.
The biggest difference for me, though, is mental. That's the really awesome part. My mind is just so much calmer and more at peace, my sex-drive doesn't own me anymore, I can actually feel emotions now instead of feeling like they're buried under the surface, I smile way more at things, and I enjoy social interaction more. The difference that it's made there is astronomical in terms of just feeling "right."
