I've fallen inconveniently ill, and without much energy to write anything too involved. But I would like to make one point.
If one starts to recognize the suffering of animals as morally significant, such that one starts to actually adjusts one's actions accordingly, it can feel like a heavy burden. When others point out to you, accurately, "It's much harder on the animals than it is on you," it's little consolation. Hearing numbers like "56 billion land animals killed per year" in worldwide animal agriculture, can be incredibly depressing, and discouraging. It's also not helpful to realize, as Darwin in part revealed, that wild animals often have very brutal lives.
But there is a flipside to the badness of the suffering, which is of course the goodness of the pleasure that animals can experience. The suffering almost undoubtedly outweighs the pleasure, so it's (appropriately) not a complete comfort, but it shows something important about the expansion of the moral community. There's a lot more suffering in the world to worry about before one seriously took animals into account, but there's a lot more joy in the world as well. So recognizing the importance of non-human animal life is not simply a burden. And that is significant.
As a sidenote, I'd like to add that there is something philosophically and phenomenologically interesting about the shift one undergoes when one accepts some version of an animal rights theory. Because of course everyone knows that there's a lot of animal suffering in the world. It just has a different effect on the individual once they conclude (or perhaps, by virtue of them concluding) that animals have rights. (This point is especially vivid if you consider that many people who work in animal agriculture have even more direct knowledge of the kind and amount of suffering that goes on, and yet are likely not heavily weighed on because they assume animals do not have significant moral rights. On the other hand, I have to assume that it weighs on them in some sense.)
My thoughts are not all that clear on this point, but I intend to think about it further.