For people short on time (like me), I’ll point out this article brief, highly skimmable, and full of self-explanatory graphs. It also briefly explains reasons the researchers think Veganuary succeeds where similar efforts have failed (i.e., the time of year and social experience).
At first glance, the evidence presented here seems quite impressive for near-term reductions in animal product consumption. However, it may not be as impactful as it initially seems (see the comments on this post).
To the extent that it is effective, I’m unsure whether it indicates significant moral circle expansion because many participants may already include farmed animals in their moral circle, meaning they aren’t expanding their moral circle, but instead accepting the implications of their moral circle (which may be a distinction without a difference, as both produce equally-good long-term consequences).
Where are you getting that it didn't decrease meat sales? I see nothing in the article pointing to that and they also point out that aggregate meat sales have been calling.
I would be extremely skeptical that vegan consumption could go up a lot without meat consumption going down, since that would imply people are just consuming a lot more food in aggregate compared to previous years, which seems unlikely.